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Star Sapphires & Rubies

The star gemstones of the corundum family are either star sapphires or rubies.  Under light these gemstones exhibit asterism [Ref 1] in showing either a six rayed or twelve rayed star.  The starred gems present a beautiful and novel appearance and are highly desired by artisans, those who wear them and collectors.  In this blog I’ll briefly describe what causes the stars and how the stars are formed by directional reflection of light.  I’ll also present a gallery of star sapphires and star rubies exhibiting a range colors and hues.

Figure 1.  Blue star sapphire with an almost ideal star [Ref 2].

ORIGIN OF THE STAR

The presence of a six-rayed or twelve-rayed star figure on the surface of a starred sapphire or ruby is caused by the scattering of light from oriented needle-like included micro-crystals of the mineral rutile, [Ref 3]. In some gemstones, this same effect is due to oriented needle-like microcrystals of the minerals hematite or ilmenite, [Ref 4].

In describing the orientation of the included crystals, within the crystal, the diagrams of cross sections of the two prism forms, shown in Figure 2, are helpful.  The view is down the c-axis of a corundum crystal [Ref 5].

In the diagram below, the faces of the first order prism are labeled “m” and those of the second order prism are labeled “a”.  The included rutile crystals form on internal planes, parallel to the “m” faces, and crystals of hematite/ilmenite form on internal planes, parallel to the “a” faces.  The six-fold orientation of the rutile crystals around the prism, is shown in the micrograph in Figure 3.  The star sapphire was cut with six facets, centered on the c-axis of the crystal, thus exposing the rutile crystals.  An enlarged view of rutile crystals is given in Figure 4. 

Because of the symmetry of the arrangement of the included rutile, hematite, and ilmenite crystals around the c-axis, the star sapphire or ruby is cut from the crystal so that the star is centered at the apex of the cabochon as shown in Figure 2.

The angle between the face of the first and second order prism is 30 degrees. 

The presence of crystals of both rutile and either hematite or ilmenite on faces separated by 30 degrees results in a star with twelve rays separated by 30 degrees as shown in Figure 5.

Figure 2.
Figure 3.  A star sapphire with six facets to show the six directions of alignment of the crystals [Ref 6].
Figure 4.  Rutile crystals in the basal plane of a star sapphire [Ref 7].
Figure 5.  Plan for cutting a star sapphire [Ref 8]. In the figure a ray of a star is referred to as a chatoyant band
 [Ref 9].
Figure 6.  Twelve-rayed black star sapphire [Ref 10].

How Light Scattering from Needle-like Crystals Creates a Star

A needle-like crystal scatters light incident directed perpendicularly to its length.  The crystals diameters are in the micron range as shown in the microscope images in references 11-13 and listed in Table 1.  This size range is just larger than the wavelengths of visible light in the 400-700 nm (0.4-0.7micron) range and equations formulated by Mie describe the intensity of the light with forward and reverse scattering angles as shown in Figure 7.  It is constructive and destructive interference, between the light waves incident on the boundaries of the needle, that result in the scattering of light, both in the original direction of the light, and in the reverse direction, as shown in Figure 7.  It is the light scattered in reverse directions towards the viewer that forms the visible star. 

TABLE I: Diameters/Widths of Included Rutile/Hematite Crystals in Sapphire/Ruby

Reference NumberFigure NumberSize Range
111A~ .5-.7 microns
1211~ .5-1.4 microns
138 A, B, C~ 1-1.5 microns

Figure 7.  Diagram showing the angular variation of the intensity of light scattered by a needle-like crystal.  The direction of light is incident perpendicular to the length of the crystal [Ref 14; 15]. 

Irregularly Shaped Stars

Two irregularities are lack of uniform intensity along a ray of the star, and a star with a geometrical offset.  As opposed to a star with uniform intensity along each ray of the star, as shown by the gem in Figure 1, other star sapphires exhibit irregularities within the rays of the star as shown in the example of Figure 8.  The variation in intensities reflect varied densities of crystals along the length of the ray.  Varied conditions during crystal growth can affect the concentrations of elements required for the formation of rutile or hematite/ilmenite crystals.  With repetitive changes in growth conditions, gaps in a ray can occur, or a region of lesser concentration of crystals with diminished intensity can occur.  Often consecutive regions or growth lines with different color or hue can also be seen.

Figure 8.  Star sapphire exhibiting an imperfect star due to various conditions of crystal growth [Ref 9]. In some regions of growth conditions in the ruby such as temperature and concentration of titanium were such that no rutile crystals formed and in others at lower densities [Ref 16].
Figure 9.  Star sapphire with irregularly shaped star [Ref 17]

Dual Star Sapphires and Rubies

There are two types of starred sapphires or rubies which exhibit twin stars. 

Dual stars from twinning

A blue dual-star sapphire is shown in Figure 10.  Two offset six-rayed stars in parallel orientation are present.  The two stars on the cabochon result from the reflection from multiply stacked, adjacent microns-thick, regions of the crystal with two adjacent regions being twinned in respect to each other, as shown in Figure 11.  The orientation of the rutile crystals in each member of a twin are different; according light scattering results in the formation of two stars on the surface of the crystal, as shown in Figure 12.

(The star sapphire shown in Figure 10 displays irregularities in the shapes of the rays in the form of “jogs”.  These arise from local regions in which the crystal lattices have different orientations from one another).

Figure 10.  Dual star sapphire [Ref 19].
Figure 11. Multiple twinning in dual star sapphire or ruby [Ref 18].

Dual-Color stars

The dual-color, starred gemstone, presents the images of two stars, identical in shape, with one white and the other the color of the gemstone.  Typically, the gemstone is transparent.  When the gemstone is viewed obliquely the rays of the two stars are offset with rays parallel on the surface of the gemstone, as shown in Figure 12.  With a decreasing viewing angle, the offset decreases. When the gem is viewed perpendicular to its center, the two stars are superimposed.  This behavior indicates the colored star to be the reflection of the white star from the back surface of the gemstone, with the reflected image picking up the color of the gemstone.

Figure 6. Purplish pink sapphire from Myanmar in a view to the curved dome (d) (left column) and the curved base (b) of the cabochon (right column). In the top row, the white arrow indicates the direction of light, and the eye symbolizes the observer receiving the reflected light. In both orientations, the body colored six-rayed star is focused within the cabochon (middle row), and the white star is focused slightly outside the stone, between surface and observer (bottom row). The sample measures 7.0 mm in diameter, with a thickness of 3.5 mm, and weighs 1.62 ct Photos by K. Schmetzer.

Figure 12.  Pink dual color star sapphire [Ref 18].

Gallery of Star Sapphires and Rubies

For this gallery, I’ve chosen gems which held special appeal to me.  I hope you find them beautiful as well.

Figure 13.  Blue star sapphire from Sri Lanka [Ref 21].
Figure 14.  Black star sapphire from Thailand [Ref 22].
Figure15.  Yellow star sapphire [Ref 23].
Figure 16.  Orange star sapphire [Ref 24].
Figure 17.  Pink star sapphire [Ref 25]
Figure 18.  Star ruby [Ref 26].

Diamonds IV

Lattice Defects, Impurities and Color

As stated in references 17, 18 of my DIAMONDS II blog, impurity atoms and lattice vacancy defects are responsible for the coloration of diamonds [Ref 1].  A lattice vacancy (V) without a carbon atom in the diamond lattice which partners with one to four neighboring nitrogen atoms (N), in colored diamonds, as well as the space taken by an adjacent pair of them partnering with a nickel atom (Ni), are present in colored diamonds. These atom-vacancy structures are shown in Figures 1 to 6 of this Blog.  Boron atoms (B), substituting for carbon atoms in the crystal lattice, give rise to the blue color.  Hydrogen atoms, possibly associated with lattice vacancies, and nitrogen atoms may also be responsible for imparting color [Ref 2].

Figure 1.  The N3 -nitrogen Center [Ref 1].  The vacancy is the space at the tips of four unfilled carbon bonds.
Figure 2.  H3-nitrogen Center [Ref 2].  The vacancy is at the tips of the four unfilled nitrogen bonds.
Figure 3.  The NVO-nitrogen Center [Ref 3].
Figure 4.  The NV-1-nitrogen Center.  Only the carbon bond of one carbon atom is unfilled [Ref 4].
Figure 5.  Boron substituting for a carbon atom in diamond.  [Ref 5].
Figure 6.  Nickel atom occupying the space of a di-vacancy
[Ref 6].

References for the N3-nitrogen center, the H3-nitrogen center, the NVO-nitrogen center, the NV-1-nitrogen center, the boron atom substation for a carbon atom, and the Nickel Di-vacancy center are described respectively in Refs 3, 4.

The colors of diamonds associated with these lattice defects and impurities are referred to in my DIAMONDS II Blog and shown below in Table I.

TABLE I

DEFECT/IMPURITY                     DIAMOND COLOR
N3-nitrogen centerYellow
H3-nitrogen centerGreen
NVO-nitrogen centerPink
NV-1-nitrogen centerPink
Boron atom substation for carbonBlue
Nickel Di-vacancyGreen
Uncertain defect due to HydrogenGrey-brown, Yellow, Pink

Diamonds III

A Gallery of Natural Diamond Crystals

The gallery includes clear crystals and those exhibiting a range of colors (Figures 2-13), a range of shapes (Figures 2-15), examples of inclusions found in Diamonds (Figures 1-3), examples of shaping and etching by resorption (Figures 3-15), and examples of twinned diamonds (Figures-14,15).  Well-developed crystal forms, typical of diamonds, are shown in Figure 1. References to the impurity and structural defects as causes of differently colored diamonds, to twinning of diamonds, and their deposition including resorption can be found in DIAMONDS II.

Figure 1.  Crystal forms of native diamonds and the tetrahedral arrangement of carbon atoms in the crystal lattice [Ref 1].
Figure 2.  Pink cubic diamond crystals, Argyle mine, East Kimberley region, Western Australia [Ref 2]. Slight rounding and etching due to resorption is present [Ref 3].
Figure 3.  Colorless rounded dodecahedral diamond crystal, location not given [Ref 4].  Some rounding due to resorption is present [Ref 3].
Figure 4.  Rounded yellow octahedral diamond crystal (The Oppenheimer Diamond), Kimberley region, Republic of South Africa [Ref 5].  Rounding due to resorption is present [Ref 3].
Figure 5.  Octahedral diamond crystal with included garnet crystal [Ref 6].  Slight rounding of the octahedron’s corners is present due to resorption [Ref 3].
Figure 6.  Green clinopyroxene crystals in an irregular brown diamond, unstated location [Ref 7]. A large degree of rounding due to resorption is present [Ref 3].
Figure 7.  Sulfide mineral inclusions in octahedral diamonds and in a twinned crystal, Diavik mine, Northwest Territories, Canada [Ref 8].  A slight degree of rounding by resorption is present [Ref 3].
Figure 8.  Hydrogen inclusions appearing as a brown cloud in a diamond formed in liquid metal in the deep mantle of the earth [Ref 9].
Figure 9.  Diamond octahedrons shaped and etched by resorption [Ref 3], [Ref 10].
Figure 10.  Rounded brown diamond octahedron [Ref 3][Ref 11]
Figure 11.  Rounded green diamond octahedron [Ref 3],
[Ref 12].
Figure 12.  Rounded grey diamond octahedron  [Ref 3] [Ref 13]
Figure 13.  Rounded blue diamond rough [Ref 3]
[Ref 14] with cut gem.
Figure 14.  Rounded twinned crystal with some rounding.  [Ref 3]
[Ref 15] 
Figure 15.  Sixling diamond crystal formed by two interpenetrating diamond twins which are mutually twinned on octahedral planes; some rounding by resorption is present.  [Ref 3] [Ref 16]

Diamonds II

In this blog, my second on diamonds, I’ll discuss the physical and chemical properties of a diamond, then its optical properties in relation to its crystal structure and chemistry, and then their formation and associated geology.  

CRYSTAL STRUCTURE AND PROPERTIES OF DIAMOND

To understand the physical properties of the diamond it’s important to understand its crystal structure, which underlies its strength, and its other mechanical properties.  To understand the sources of the various colors observed in diamonds, it’s necessary to relate them to localized defects in the periodic array of carbon atoms within the crystal lattice, as well as to the impurity atoms in the lattice.

Physical Properties of Diamonds

In this section those physical properties affecting diamond as a gemstone are described.  Diamond, unique among minerals, is comprised of only one element, carbon.  In the crystal lattice of diamond, the carbon atoms can be pictured as being located at each of the four corners of a tetrahedron, embedded in a cube, as seen in Slide 7 of [Ref 1] and shown in Figure 1. 

Figure 1.  Tetrahedral arrangement of carbon atoms
in a Diamond.

Extension of the arrangement of the four carbon atoms in the tetrahedron to the lattice, results in the arrangement of carbon atoms in the cubic unit cell of the crystal lattice, as also shown in Figure 1.  Bonds between the carbon atoms extend throughout the diamond crystal, as shown in Figure 3.  Their strength and arrangement underlie the physical properties of the diamond.

Strength of a Diamond

With a hardness of 10 on the Mohs Hardness Scale [Ref 2], and a relative hardness scale of minerals in the range of 1-10, diamond is ranked as the hardest of minerals.  The scale is approximately logarithmic. As a quantitative example, the tensile strength, or resistance to pulling, by natural diamonds has been measured within the range 2.8 – 2.93 GPa (452,776-473797 psi) [Ref 3].  In parallel, these large values of tensile strength are exceeded by values of the compressive strength of diamond, in the range 10-20 GPa (1,617,057-3,234,114 psi) [Ref 3].  This large compressive strength underlies the uses of diamond windows in the diamond anvil cell, as shown in Figure 2. Used for exerting very large pressures on a material for study of its properties [Ref 4] Recently the formation of metallic hydrogen, by breakdown of molecular hydrogen, (under familiar conditions, a gas), at a pressure of 495 GPa (71.700,000 psi) was achieved using anvils of a specially treated synthetic diamond [Ref 5].  Interestingly, this result supports the hypothesis that the core of the planet Jupiter may be comprised of liquid metallic hydrogen [Ref 6].

The large strength values derive, in part, from the strength of the carbon-carbon bond in the diamond, having the energy of 346 (kilojoules)/(mole of carbon), [Ref 7], and from the pattern of orientation of carbon-carbon bonds within the lattice, with the exception of the pattern of those bonds crossing the octahedral planes of a crystal, along which the crystal can be cleaved. As seen below.

Figure 2.  Diamond anvil cell for studies of the properties at ultrahigh pressures.  Both anvils and the gasket for confining the specimen are shown.  Application of pressure along the axis of the cell is transmitted to the specimen [Ref 4]. 

Cleavage in a Diamond

Despite the strength of the carbon bonds in the diamond, its lattice does possess a specific set of planes of weakness along which the severing of the bonds requires the least energy.  In all other directions, with the application of shear force, the diamond lattice will resist compression or tension.  Diamonds exhibit a perfect and easy cleavage or separation along planes parallel to any one of the eight faces of an octahedron, as can be seen in Figure 3 [Ref 8].  In any other direction the number of bonds necessary to be broken for cleavage are larger than along this octahedral plane. This feature underlies the use of intentional cleavage of a rough diamond in removing material prior to further steps in preparing a gem [Ref 9].

Figure 3.  Octahedral cleavage planes in diamond.

Visualizing cleavage in diamond

In Figure 4, a partial cleavage is seen to be exactly oriented, parallel to the opposing octahedral face [Ref 10].  Additionally, the view of the diamond lattice parallel to an octahedral plane and the slightly tilted view in Figure 3, shows that the minimum possible number of carbon-carbon bonds per unit area cross this plane, thus, requiring the least possible energy for cleavage.

Figure 4. Octahedral cleavage in diamond.

As a caveat, because of its easy and perfect cleavage, and despite its hardness, toughness or tenacity, the ability to resist fracture is only fair to good, because its easy cleavage renders it susceptible to breaking.

Twinning in Diamonds

Twinned crystals of diamond present a unique form, quite unlike that of a single crystal.  Conceptually, twinning in diamonds occurs between the octahedral faces of two crystals, as shown diagrammatically in Figure 5 [Ref 11].  This twin form is also known as a macle.  The reentrant angle at the base of the triangular faces of the macle is shown on the twin in Figure 6.  Two macles can pair as shown in Figure 7, to form a Star-of-David twin [Ref 12].

Figure 5. Octahedral twinning (Spinel Law twinning) in diamonds [Ref 11]
Figure-6.  Twinned diamond crystals (a macle).  In this twin two octahedral faces face each other [Ref 11].
 
 
Figure 12.  Star of David Twin, Northern Cape Province, RSA [Ref 12].  The two macles are twinned on mutual octahedral planes.

Optical Properties of Diamond

The appreciation of diamonds as gemstones, lies in their sparkling brilliance, and in their colors, which span the rainbow.  Their brilliance is due to their large refractive index and dispersion of colors.  Their colors are due to the presence of impurities and defects in their crystal lattice.

Refractive index and dispersion

The refractive index of a gemstone is a measure of the magnitude of the angle at which light is bent, (refracted), as it enters from the outside [Ref 13].  If the refractive index of the material at which the light enters exceeds that at the exterior, the light will bend more towards the perpendicular (Figure 1 in Ref 13)

The brilliant appearance of a diamond, compared to other gemstones, stems from its larger refractive index and degree of dependence on the wavelength of the incident light.  The brilliance, or dance of colors seen on a cut diamond stems from its large dispersion, which is a measure of splitting light into its constituent colors after entering the gemstone [Ref 14].  Such dispersion can be demonstrated by the rainbow of colors which exit from a glass prism, as shown in Figure 13.  As a uniform measure for all gems, the dispersion is calculated as the difference in refractive index values measured with light at 430.8 in the violet and nm ad 686.7 nm in the red region [Ref 14].  The difference of the two values gives the dispersion of the diamond d = 2.451-2.407 = 0.044 [Ref 14].  This value is very large compared to other gemstones.  For example, this value far exceeds the value of 0.014 for difference in the refractive index of light determined at two wavelengths, 430.8 nm in aquamarine (and also for emerald, heliodor, and morganite), the value of 0.018 for corundum (and also for ruby and sapphire) [Ref 15].

Figure 13. Dispersion of white light into colors.

The angle at which visible light is bent as it enters the diamond depends directly on the wavelength-dependent refractive index.  The cut of the gem, or the overall placement of the facets on the rear, sides and face, is designed to optimize the play of colors developed within the gem by dispersion for the viewer.  The facets must be placed at angles calculated to optimally reflect the light back into the gemstone and position it to escape from the face of the gemstone. The American Standard Brilliant Cut [Ref 16], among others, maximizes the total amount of light and the play of its colors, leaving the gem as shown in Figures 14 and 15. 

Figure14.  The angles of the facets of the gem comprising the bottom of the gem are determined to maximize the amount of light re-emerging from the stone [Ref 17].
Figure 15. “Fire in colors by dispersion”.

Impurities, Crystal Lattice Defects, and Color of Diamonds

Many of the structural impurities, which are sources of colors in diamonds, are described in the Shigley Chart [Ref 18].  In the list of color sources, nitrogen atoms are labeled N, as impurities substituting for carbon atoms in the diamond lattice, alone or associated with an adjacent lattice vacancy labeled V, and are seen to cause many of the colors which range over the green to red portion of the visible spectrum.  Additionally, boron atoms, labeled B, as impurities substituting for carbon atoms, impart a blue color, while a single lattice vacancy labeled V, imparts a green color. Groups of vacancies can impart a yellowish-brown color.  Also, interstitial hydrogen atoms lying in spaces between carbon atoms, impart a yellowish-brown color.  In another study, an interstitial nickel atom as an impurity, (with an associated large-sized split-vacancy Ni-2V), was found to impart a green color [Ref 19]. 

The Formation of Diamonds [Ref 20]

Diamonds are formed within the earth’s mantle at remarkable depths of 80 kilometers and greater, and at notable pressures of 2.6 Gigapascals or 377098 psi and greater, as shown in Figure 22.  They are violently projected to the surface by magma rising at velocities of 200 meters per second, which fractures the overhead rock and forms a crater at the surface containing a plug of the host rocks of diamonds, as shown in Figure 23.  The major host rocks are kimberlite [Ref 21] or lamproite [Ref 22] as shown in Figures 23 and 24.  Both rocks are breccias formed of cemented fragments, which resulted from the rock fracturing.  Figure 27, taken of the Diavik mine in Canada, shows the circular contours of two kimberlite ore bodies on the surface [Ref 20].

Figure 22.  Pressure and depth relationships for the formation of diamonds [Ref 20].
Figure 23.  Formation of a kimberlite or lamproite ore body [Ref 20].
Figure 23.  Kimberlite comprised of cemented rock fragments
[Ref 21].
Figure 24.  Lamproite comprised of cemented rock fragments
[Ref 22].
Figure 27.  Workings of the Diavik Diamond Mine [Ref 20].

Resorption with etching and rounding of the surfaces of diamonds can occur due chemical attack by the acidic environment, due to the presence water (H2O), and oxidation of the diamonds carbon to form carbon dioxide (CO2) within the melt of the magma [Ref 23].  Examples of the rounding of the shapes of octahedral and cubic crystals with increasing resorption are shown in Figure 28.

Figure 28.  Shape changes of diamonds indicating rounding of their shapes due to resorption increasing with pressure at depth and temperature [Ref 23].

Diamonds I

Having greatly enjoyed writing my last blogs on garnets as gemstones and mineral specimens, topics different from my long-time interests in the ore-forming minerals, I am starting a series of blogs in the same vein on the precious gems: diamonds, emeralds, rubies, and sapphires. 

In the first three blogs the topic is diamonds.  In this first blog, I will describe some famous diamonds and their histories, and follow up with a gallery of diamond jewelry and diamond-decorated art objects from over the years.  In the second blog I will briefly discuss the crystal structure of diamonds, which underlies their physical and optical properties, including their brilliance and colors, and how diamonds are formed deep within the earth’s mantle and then ejected to the surface.  In the third blog I’ll end the series on diamonds with a photo gallery of natural uncut diamond specimens, emphasizing varieties of crystal forms and colors. 

THE BEAUTY AND ALLURE OF DIAMONDS

Not only do diamond gemstones display a remarkable sparkling brilliance, their colored varieties, perhaps unexpectedly, span the spectrum of the rainbow (Figure 1).  These properties underlie their being esteemed over the years by collectors and wearers of these gems.  Diamond’s beauty and early rarity due to limited sources, along with their aura of great wealth and power, underlay their being worn as crown jewels, being set as the gemstones of royals and glitterati, and being set in jewelry, which are also art objects.  Some famous gemstones and other art objects are described below.

Figure 1.  Colors of diamonds. 

The Koh-i-noor Diamond [Ref 1]

The rough stone was mined in the alluvial sands in the Golconda, the ancient diamond-mining region of India [Ref 2].  Its name is Persian for “Mountain of light”, attesting to its brilliance.  Prior to entering the written record, it was shown in 1628 as being set at the top of the gem-laden throne of the Mughal ruler Shah Jahan (Figure 1) [Ref 3]

Figure 2.  Peacock Throne of the Mughal ruler Shah Jahan [Ref 3].

Following its possession by Mughals, it was taken from India in 1739 by the Persian ruler Nadar Shah among the spoils of war.  The gem remained in what is now Afghanistan until, after decades of fighting, it was returned to India by the Sikh ruler Ranjit Singh in 1813.  After Singh’s death in 1839 the gem passed from ruler to ruler before being possessed by a boy, Duleep Singh and his mother, Rani Jindin.  Following her imprisonment by the British, who, as the British East India Company, expanded into central India, ten-year old Duleep was forced to give away the Koh-i-noor and his claim to sovereignty.  From there the diamond came into the possession of Queen Victoria after it was recut to enhance its brilliance (Figure 3) [Ref 4].  It was set in a brooch, (Figure 4) [Ref 5], and became part of the British Crown Jewels [Ref 6].  After resetting, the diamond now resides at the front of the crown of the Queen Mother, mother of Queen Elizabeth II (Figure 5). 

Figure 3.  The Koh-i-noor diamond before and after re-cutting [Ref 4].
Figure 4.  Queen Victoria wearing the brooch with the recut Koh-i-noor diamond [Ref 5].
Figure 5.  Queen mother’s crown with the round brilliant recut Koh-i-noor Diamond [Ref 6].

The Cullinan Diamond [Ref 7]

More recently found than the Koh-i-noor, the Cullinan diamond (Figure 6) was found in 1905 at the, (then called), Premier Mine in Johannesburg, South Africa.  At 3106 carets (1.371 lbs.) it was the largest rough diamond ever found, until the discovery of a 1109 caret diamond in Botswana.  Cutting it provided seven gemstones (Figure 7) including the Cullinan I and Cullinan II diamonds.  These were set respectively in the Sovereign’s Scepter (Figure 8) and in the Imperial State Crown (Figure 9), both among the British Crown Jewels.

Figure 6.  The rough Cullinan Diamond as discovered [Ref 7]. 
Figure 7.  The major gems from the Cullinan diamond [Ref 8].
Figure 8.  The Sovereign’s Scepter of England with the Cullinan I Diamond [Ref 9].
Figure 9.  The Imperial State Crown of England with the Cullinan II diamond, the St. Edward’s Sapphire and the Black Prince’s “Ruby” (actually a red spinel) are also set on the crown [Ref 10].


The Hope Diamond [Ref 11]

Reportedly cursed, after being stolen from the eye of a holy Hindu statue, the original 115.16 ct stone was likely purchased by Jean-Baptiste Tavener, a diamond merchant, from the Kollur Mine in Golcanda, India and sold in 1668 to King Louis XIV of France, and became known as the “French Blue”.  It was recut to a 67 ct gem by the court jeweler [Ref 11].  During the French Revolution the French Blue was stolen.  Later the stone changed hands and went to Henry Phillip Hope. It is now famously known as the Hope Diamond.  After a series of purchases, it was donated to the Smithsonian Institution in 1958 where it resides today.  After its acquisition by the museum the weight of the stone was determined to be 45.52 carats. Its intense blue color is seen in Figure 10. 

Figure 10.  The Hope Diamond [Ref 11].

The Orlov Diamond [Ref 12].

The rough diamond ancestor of the Orlov Diamond was mined in the Kolliur Mine in the Golcanda region of India [Ref 12] and weighed 787 carats. Following it’s residence as an eye of a deity, in a temple on the island of Srirangam, in South India, it made its way to Amsterdam, and around 1768 was sold to Count Grigoryevich Orlov, a Russian nobleman and officer who helped Catherine the Great overthrow Peter III and gain the throne of Russia.  Catherine had the diamond mounted on the Imperial Scepter (Figure 11).  The diamond has a non-traditional shape, resembling half of a bird’s egg.  It is a white diamond [Ref 13] with a bluish-green tint and weighs 189 cts.

Figure 11.  The Olov Diamond atop the Russian Imperial Scepter [Ref 13].

The Tiffany Yellow Diamond [Ref 14]

This rough diamond, weighing 287 cts, is the largest yellow diamond ever mined. Purchased by Charles Lewis Tiffany, founder of Tiffany & Co., in 1877.  It was fashioned into a cushion-cut brilliant, weighing 128 cts.  The gemstone was fashioned into a diamond necklace (Figure 12) and worn by Audrey Hepburn in promotional photos for the film Breakfast at Tiffanys.  Later the gemstone was mounted to create the whimsical “Bird on a Rock” brooch (Figure 13).  The stone was reset into a diamond and platinum necklace in 2012 to commemorate the 175thanniversary of Tiffany & Co.

Figure 12.  Necklace with the Tiffany Yellow Diamond as worn by Audrey Hepburn [Ref 16].

Figure 13.  Bird on a Rock brooch with Tiffany Yellow Diamond [Ref 15]
 

Gallery of Diamond Jewelry as Art Objects

Figure 14.  Mukut (Crown) head ornament of Lord Vishnu with diamonds and other precious stones (1300-1500) [Ref 17].
Figure 15.  Mughal turban ornament with diamonds, rubies, emeralds [Ref 18].
Figure 16.  (Inscribed Shah diamond) [Ref 19]
Figure 17.  Indian Prince Diamond necklace [Ref 20].  The triangular gems are worked twinned crystals (Macles).  See gallery of specimens in the Diamonds III blog.
Figure 18.  Nephrite jade box with diamonds, rubies, and emeralds [Ref 21].
Figure 19.  Art Deco diamond, emerald, and platinum bracelet, ca. 1915 [Ref 22].
Figure 20.  Art Deco Diamond, Coral, Onyx and Platinum Ring [Ref 23].

Fluorite Art Works & Jewelry

Artworks and jewelry have been created from fluorite since ancient times, such as the carved fluorite statues discovered in Crittenden County, Kentucky and shown in Figure 1 [Ref 1]. The statues reflect the Middle Mississippian Culture extending approximately 800 Ad to 1600 AD. The second statue from the left, known as the Tolu Statue is dated to 1150-1200 AD. The fluorite for these carvings undoubtedly came from Southern Illinois and the adjacent region of Kentucky, which were centers of fluorite mining from the early 1800s until 1995.

Figure 1. Carved fluorite statues dating from the Middle Mississippian Culture era [Ref 1].


Another example of art work in fluorite is the beautiful antique carved statue of Quan Lin, the Chinese goddess of mercy and compassion displayed in Figure 2 [Ref 2, Ref 3].

Figure 2. Fluorite statue of Quan Lin the Chinese Goddess of mercy and compassion [Ref 2, Ref 3].

Crafting art works and gemstones of fluorite demanded the artisan’s appreciation of its low hardness of 4 on the Moh Hardness Scale, and its perfect, (and easy) octahedral cleavage [Ref 4], as well as their great artistic skill. For example, faceting the fluorite gemstones, shown in Figure 3, required they determine the orientation of the fluorite crystal to minimize stress along the eight cleavage planes in the crystal, while cutting the stone. Larger artworks were carved or formed using polycrystalline fluorite so that the interlocking of the multiple crystal faces prevented propagation of cleavage while working the art object. Careful examination of Figures 1 and 2, and the artworks in the following figures, will disclose their polycrystallinity.

Figure 3. Faceted fluorite gemstones [Ref 5].

Cabochons often are formed of banded fluorite for their beautiful display of multiple colors as shown in Figure 4.

Figure 4. Cabochons of banded fluorite [Ref 6]. The interlocking of micro-crystals in the bands and at their interfaces prevented cleavage of the stone during cutting.

GALLERY OF JEWELRY AND ART WORKS  –  In the following gallery diverse, beautiful examples of artworks and jewelry representing the creative skills of the artisans are presented in Figures 5-16.

Ref 1. http://www.lithiccastinglab.com/gallery-pages/2011junetolustatuepage1.htm

Ref 2. https://www.rubylane.com/item/1061514-0015-7084/Antique-Quan-Yin-Carved-Fluorite-Stand

Ref 3. https://en.wikipedia.org/wiki/Guanyin

Ref 4. https://www.mindat.org/min-1576.html

Ref 5. https://www.gemsociety.org/article/fluorite-jewelry-gemstone-information

Ref 6. https://www.healingcrystals.com/Cabochons_-_Rainbow_Fluorite_Cabochon__India_.htm

Ref 7. https://www.gemsociety.org/article/fluorite-jewelry-gemstone-information/

Ref 8. https://en.wikipedia.org/wiki/Blue_John_(mineral)

Ref 9. https://www.ebay.co.uk/itm/ANTIQUE-CHINESE-HARDSTONE-BLUE-JOHN-SNUFF-BOTTLE-18TH-C-/202054494962

Ref 10. https://www.pinterest.com/pin/536913586810773819/?lp=true

Ref 11. https://www.pinterest.ca/pin/158259374378896038/

Ref 12. https://www.gemsociety.org/article/fluorite-jewelry-gemstone-information/

Ref 13. http://www.antiques.com/classified/Asian-Antiques/Asian-Decorative-Arts/Antique-Fluorite-Carving-Of-Grape-Cluster

Ref 14. https://beadage.net/gemstones/fluorite/

Ref 15. http://www.incense-burner.com/index.php?st=c&mat=Jade/Agate/Fluorite

Ref 16.https://www.florencejewelshop.com/product/green-silver-fluorite-necklace-of-fluorite-sterling-silver-with-a-silver-magnetic-clasp-lomboks-dream/#iLightbox[product-gallery]/0

Ref 17. https://www.ebay.com.my/b/Toucan-Collectibles/165275/bn_3028968

Ref 18. https://www.alibaba.com/product-detail/Fluorite-Gemstone-Silver-Necklace-925-Sterling_114584988.html

Ref 19.  https://www.pinterest.com/pin/570760952750804859/?lp=true

Fluorite, A Collectors Favorite

The rich, often “electric” rainbow colors of fluorite specimens from world-wide locations, the frequent appearance of different colors within regions, bound with the lattice planes within a crystal, and the sculptural appearances of stepped cubic and octahedral crystals, as shown in Figures 1-5, make fluorite a favorite among collectors. 

In this blog I’ll describe those structural properties of fluorite that are relevant to lapidary practices, and the sources of its’ color, (Figure 8).  I’ll describe how the regions, or zones, of different colors within a fluorite crystal develop. Also, because of their beauty, I am including a gallery of the beautiful fluorite specimens from around the world, that I found as I searched the web.

Figure 1. Stepped color-zoned fluorite crystals with boundaries parallel to the cubic faces of the crystals, Ozark-Mahoning Mione, Cave-in-Rock sub-District, Illlinois-Kentucky Fluorite District, Hardin County, Illinois [Ref 1]
Figure 2. Color-zoned fluorite on quartz with diagonal boundaries between the green and purple regions directed along dodecahedral planes in the fluorite crystals, Kudubis Farm 19, Karibib District, Erongo Region, Namibia [Ref 2].
Figure 3. Color-zoned fluorite with boundaries along diagonal dodecahedral planes within the cubo-octahedral crystal, White Water, Pigeon Rock Mountain, Mourne Mountains, County Down, Northern Ireland, UK [Ref 3].
Figure 4. Stepped fluorite cubic crystal, Spain [Ref 4]
Figure 5. Stepped fluorite cubo-octahedron, Shangbao Mine, Leiyang, County, Hengyang Prefecture, Hunan Province, China [Ref 5].

Properties of Fluorite [Ref 6]

Crystal Structure and Crystallography

The crystal lattice of fluorite exhibits cubic symmetry with the calcium and fluorine ions positioned within the basic unit cell, as shown in Figure 6. The four fluorine ions F-1are positioned centrally in the unit cell with the calcium ions Ca2+distributed at the corners and the center of each face of the unit cell. In this array, each calcium ion has two fluorine ions as neighbors. Imperfections in the lattice underlie the presence of color in fluorite, as described below.

The lattice structure of fluorite results in its crystallizing in the isometric crystal system. Fluorite typically exhibits the basic crystal forms of the cube, octahedron, and rhombic dodecahedron shown in Figures 7 & 8. Fluorite also exhibits ball-like or grape-like botryoidal forms, but only rarely. (Figure 9)

The Mohs hardness of Fluorite is 4 and it exhibits a perfect cleavage on octahedral planes and less-well developed cleavage or parting on dodecahedral planes. A cleavage octahedron is shown in Figure 12 [Ref 14]. The ready cleavage and softness of fluorite requires the lapidarist to minimize the generation of stresses during the cutting and polishing of a stone.

Figure 6. the crystal lattice of fluorite [Ref 6].

Typical Crystal forms of Fluorite

Typical crystal forms are the cube, oxctahedron, and dodecahedron; fluorite also occurs in botryoidal form.

Figure 7. Cubic crystals of fluorite, Las Causses, France [Ref 7].
Figure 8. Fluorite octahedrons on smoky quartz, Argentiere Massif,
Mont Blanc, Chamonix, France [Ref 8]
Figure 9. Cubic fluorite crystal with edges modified by dodecahedral faces, La Cabana, Asturias, Spain [Ref 9].
Figure 10. Botryoidal fluorite with calcite on quartz, Mahodari Quarry, Nasik, State of Maharashtra, India [Ref 10].

Twinning in Fluorite

Fluorite forms both penetration twins [Ref 11] and Spinel Twins [Ref 12] on octahedral planes, as shown in Figures 10 and 11.

Figure 11. Fluorite penetration twin, Lake George, Florissant, Colorado [Ref 13].
Figure12. Fluorite spinel Twin, Erongo, Namibia [Ref 12]. The octahedral face of one crystal is seen on the upper surface of the twin. The other face is hidden by the crystal. The outside trace of the twin plane is irregular [Ref 14].

Cleavage in Fluorite 

Figure 13. Fluorite cleavage octahedron. Cave-in-Rock District, Hardin County, Illinois [Ref 15]. The cleavage is perfect

Colors in Fluorite 

As shown in Figure 6, fluorite can occur in colorless form. Color, as described in Table I, is induced by the presence of an impurity ion, by nanoparticles of calcium metal, which scatter light preferentially according to size, or by inclusions of organic material such as bitumen.

Figure13. Color center in fluorite [Ref 22]. REE = rare earth element
 Ref 22. Page 27, Schematic of photochromic center color center–:

Color Zoning in Fluorite Crystals 

The establishment of discrete geometric zones of color in a fluorite crystal comes from the sequestration of a specific impurity, advancing along a crystallizing plane during sectorial growth [Ref 23].  A sketched example of sectorial growth in a fluorite crystal is shown in Figure 15, which is taken from Figure 4 of [Ref 24]. In the example, growth of octahedral and cubic planes are shown.  In Figure11, showing a fluorite specimen from Namibia, growth is seen to have proceeded along both dodecahedral and cubic planes. Growth proceeded along advancing dodecahedral planes, forming the green diamond-shaped region, and along cubic planes, forming the deep green corner regions.

Figure 14. Schematic of growth of sectors in a fluorite crystal. In generation of dodecahedral and cubic sectors as in Figure 14, growth proceeds towards the four cube corners and faces, forming four-sided pyramids within the crystal [Ref 23]. 
Figure 14. Color-zoned fluorite, Okorusu mine, Namibia [Ref 24].

GALLERY OF FLUORITE SPECIMENS 

In this Gallery I’ve included specimens from France, Morocco, Germany, Spain, and Mexico, which are not as well known to collectors as specimens from England the United States.

Ref 25. http://www.trinityminerals.com/sm/frnfluor.shtml

Ref 26. http://www.webmineral.com/specimens/picshow.php?id=3141&target=Fluorite#.W7KDboUXsws

Ref 27. https://www.pinterest.com/pin/111534528254934024/

Ref 28. https://www.mineral-forum.com/message-board/download.php?id=6836&sid=5476579b13c3f45b45dcbab1a1eafaf8

Ref 29. https://www.pinterest.com/pin/592716000930774044/

Ref 30.  https://www.crystalclassics.co.uk/events/1/43rd-gem-and-mineral-show-aue/

Ref 31. http://www.johnbetts-fineminerals.com/jhbnyc/mineralmuseum/picshow.php?id=24766

Ref 32. https://picclick.com/Fluorite-Asturias-Spain-372413243400.html

Ref 33. https://www.mineratminerals.com/archive/pink-fluorite-2c-la-mina-navidad-2c-mexico-detail

Ref 34. https://www.minfind.com/mineral-520150.html

An Unusual Quartz Crystal

While at the Flagstaff Gem, Mineral & Jewelry show this past August, my fellow Coconino Lapidary Club member and collector friend, Gordon, and I spotted an unusual appearing polished quartz crystal. Upon closer inspection we spotted a geometrical array of fine lines within the crystal which looked to be perpendicular to each other. The appearance of the fine lines seemed similar to the line of incomplete cleavage shown in the octahedral diamond crystal. Mindat, [Ref 1]cites quartz as exhibiting cleavage, as well as fracture, (surprising to me upon discovery, as most sources I’ve seen describe quartz as only exhibiting fracturing), I suggested that the fine lines might be due to cleavage. Gordon purchased this crystal, (shown in Figure 1), with the intention of our investigating it to ascertain if the lines were indeed due to cleavage.

The result of our efforts was a yes. The sharp lines we observed were caused by cleavage along rhombohedral planes, as indicated by our measured and calculated angle of 87.5 degrees between the lines, agreeing well with the literature value of 86 degrees.  How we got here is described below, along with how to go about finding demonstrations of cleavage in quartz along other crystallographic planes and how to obtain values for the compressive strength of quartz from the literature.

Description of Cleavage in Quartz

In its description of quartz, Mindat states that quartz fractures exhibiting a conchoidal, (shell-like), surface [Ref 1] and, with respect to cleavage in quartz, that when “The rhombohedral cleavage {10-11} is most often seen, there are at least six others reported.”. This summary led to a web-search resulting in papers which described cleavage occurring in quartz due to shearing along lattice planes, which is due to stresses induced by very high static pressure [Ref 2].

A Cleavage Pattern Under Static Pressure

Fragments of large, single crystals of quartz, which had been embedded in veins located in the basement rocks in Madagascar during the age of the supercontinent, Gondwana, [Ref 3], were used in the study we examined. Separation of India from Madagascar incurred intense pressures and shearing of the basement rocks. Micrographs of thin sections, prepared from the fragments, disclosed two sets of parallel cleavage lines running essentially perpendicularly to each other, as shown in Figure 2 (Figure 2 of Ref 2). Orientation of the thin section by x-Ray diffraction [Ref 4] allowed assignment of cleavage, in both sets of parallel lines, to be directed along rhombohedral planes.

Are the Essentially Perpendicular Lines in the Crystal of Figure 1 Due to Cleavage? 

Comparison of the array of essentially perpendicular cleavage lines, with the two prominent essentially perpendicular lines on the left side of the quartz crystal, in Figure 1, suggested that these lines and others in the crystal might represent cleavage planes. In order to examine this possibility, the angle between the pair of approximately perpendicular lines on the left side of the crystal, were used to obtain a value for the angle between the nearly perpendicular sides. An estimate for the angle was obtained by measuring the lengths of the sides of the resulting right triangle, constructed by erecting a perpendicular to one side and computing the tangent of the included acute angle. This value allowed computation of the arctangent of the angle, giving the computed value of 87.5 degrees, which agrees well with the value of 86 degrees between the rhombohedral (r-r) faces of the drawn crystal shown in Figure 3, (Figure 3 of Reference 5),  [Ref 5]

Cleavage in Quartz Along Other Crystallographic Planes

Having obtained this result, we next researched the web further, looking for quartz crystals which exhibited evidence of cleavage along other possible planes, as given in Table 1 of Reference 5, as shown in Figure 4. Our search, of some duration, resulted in finding the two quartz crystals shown in Figures 5 and 6.

Examination of the crystal in Figure 5 disclosed two suspected cleavage lines in its lower left region. The lines intersect each other, with one being parallel to the axis of the axis parallel to the unit, or secondary prism, of the crystal and the other line intersecting it at an obtuse angle. Calculation of this angle, from measurements on an enlargement of the photo of the crystal, gave a value of 140.6 degrees which agrees closely with the angle between the unit prism and second order trigonal pyramid, (Table, Figure 5), which is shown in Figure 3 as the angle z-m = 142 degrees. In appearance both lines lack the sharpness of those of rhombohedral cleavage. This is probably due to the camera angle in the photo not being perfectly perpendicular to the cleavage lines.

Examination of Figure 6 disclosed four suspected cleavage lines perpendicular to the axis of the prism and one angled at an angle seen in the rhombohedral cleavage in Figure 1. These lines are due to cleavage along the planes parallel to the basal pinacoid (face) of the crystal (Table, Figure 5).These cleavage lines also lack the sharpness of the rhombohedral cleavage lines in Figure 1, probably due to the camera angle. [Ref 6]

Reported Values of the Compressive Strengths of Quartz and Other Silicate Minerals

We found the following references on the web. All gave results obtained at room temperature ~ 72 degrees F. In two experiments, using uniaxial quasi-static (slowly applied force)directed perpendicularly to the prism face, gave values of 2.55 Gigapascals (369,847 psi) [Ref 7] and 2.74 Gigapascals (981,719 psi) [Ref 8], respectively. In two experiments performed to determine the elastic properties of quartz results were obtained with hydrostatic pressures up to 10 GPa ( 1,740,456 psi) and 12GPa (2,088,547 psi), respectively. [Ref 9]

Figure 1. Quartz, State of Minas Gerais, Brazil [Ref 10].
Figure 2. Cleavage along essentially perpendicular sets of rhombohedral planes in quartz [Ref 2].
Figure 3. Angles between quartz faces (or crystal planes) [Ref 3].
Figure 4. Observed planes of cleavage in a quartz crystal [Ref4].
Figure 5. Polished quartz crystal, State of minas Gerais, Brazil.
Figure 6. Polished quartz crystal, State of Minas Gerais, Brazil

Quartzsite Field Trip

Our January field trip to Quartzsite saw perfect weather in the high 60’s, with Snow Birds filling the open desert with their motor-homes.

Linda and I arrived on the 17th as did John and Beth Duggan. Our rock collecting spree began at Desert Gardens, where just about every rock, mineral and gem can be found, for a price. From slabs to cabs to large medium and small rock, Brazilian amethyst cathedrals and large quartz crystal formations. Next stop was the Q.I.A Pow-Wow where more rock and minerals plus jewelry, tools and rock cutting machinery can be had. More slabs and cabs. Indoor display cases were setup, some from dealers showing their wares and some for competition to be judged.

We four made a day of it, with a final stop at Tyson Wells, a huge flee market type fair. Clothing, garden art, tools, household items, beautiful rugs, walking sticks and essential oils, and this list barely begins to cover what is available. As well as the food courts and lots of people perusing the wares.

On Saturday we met at T-Rocks, in order for any members that made the trip from Flagstaff to meet us, sadly none showed up. Maybe next year, with a little more planning and information about where to find accommodations, we’ll have a better turnout.

All in all it was a fun trip, seeing lots of friends from California and Linda and I helping with the American Lands Access Assoc. desert cleanup on Sunday morning. Then, home. It was an fun and exhausting time, but I’m sure we will be back again next year!

Martin & Linda Dougherty

Calcite II – Jewelry & Art

In this blog, art works and jewelry, dating from 2300-2400 BC to the present, are shown. All were discovered with great fun by googling the web.

Figure 1. Carved calcite (travertine) bowl, 4th -mid 5th Dynasty, 2500-2400 BC [Ref 1].
Figure 2. Carved relief calcite disc, Sumerian
2350-2300 BC [Ref 2].
Figure 3. Carved Egyptian Calcite Canopic Jar with human headcover, 17thDynasty,
1570-1085 BC [Ref 3], [Ref 4].
Figure 4. Gold earrings with calcite, cloisonne’ enamel, earthenware, and glass from tomb of Tutankhamun, 18thDynasty, 1336-1327 BC [Ref 5}
Figure 5. Egyptian carved calcite onyx lion holding a vessel, 17thDynasty, 525-404 BC [Ref 6].
Figure 6. Calcite-alabaster stele, South Arabian Peninsula, circa 3rdto 1stCentury BC [Ref 7].
Figure 7. Calcite statue of Roman emperor Hadrian, body from 4thCentury AD, base, head, and hands of gilt bronze from Italy. [Ref 8].
Figure 8. Offering vessel of calcite onyx in form of an ocelot,
Teotihuacan Culture circa 400-600 AD [Ref 9].
Figure 9. Frankish Disk Brooch, gold sheet with inlays of calcite, garnet, and glass,
lets 7th Century AD [Ref 10].
Figure 10. Brooch of sterling silver with Mexican chromian green calcite [Ref 11]
Figure 11. Carved calcite onyx apple [Ref 12].
Figure 12. Calcite and mahogany obsidian necklace [Ref 13].
Figure 13. Blue calcite egg [Ref 14].
Figure 14. Carved blue calcite bowl, Argentina [Ref 15]. 
Figure 15. Pin/Pendant of cobaltoan calcite druzy and faceted gemstones [Ref 16]

CALCITE JEWELRY AND ARTWORK REFERENCES

Ref 1. https://www.metmuseum.org/art/collection/search/543887?rpp=30&pg=2&gallerynos=103&rndkey=20150416&ao=on&ft=*&pos=55

Ref 2. https://www.penn.museum/collections/object/293415

Ref 3. https://www.dia.org/art/collection/object/canopic-jar-human-head-cover-43587

Ref 4. https://en.wikipedia.org/wiki/Canopic_jar

Ref 5. https://www.pinterest.com/pin/300333868875258132/?lp=true

Ref 6. https://www.brooklynmuseum.org/opencollection/objects/3594

Ref 7. https://www.pinterest.co.uk/pin/386605949244823995/

Ref 8. https://www.mfa.org/collections/object/hadrian-58915

Ref 9. http://www.britishmuseum.org/research/collection_online/collection_object_details.aspx?objectId=479257&partId=1

Ref 10. https://www.alamy.com/disk-brooch-late-7th-century-frankish-gold-sheet-with-copper-alloy-backing-and-inlays-of-garnet-glass-and-calcite-overall-2-14-x-1516-in-57-x-24-cm-metalwork-gold-the-dress-of-frankish-women-generally-consisted-of-a-tunic-cinched-by-a-belt-from-which-hung-an-array-of-pendants-a-wrap-or-cloak-went-over-the-tunic-shoes-and-hosiery-fastened-with-buckles-covered-the-legs-image212485417.html

Ref 11. https://www.ebth.com/items/9279883-mexican-made-sterling-silver-calcite-brooch-and-earrings

Ref 12.https://therockshed.com/polishedrock/pr1116a.jpg

Ref 13. http://www.august-veeck.de/jewellery/necklaces/calcite-obsidian/

Ref 14. https://auction.catawiki.com/kavels/10229271-large-blue-calcite-egg-17-x-11-5-cm-3-33-kg

Ref 15. https://fineart.ha.com/itm/lapidary-art/carvings/blue-calcite-bowlandes-mountainsargentinasouth-america/a/5324-72259.s

Ref 16. https://www.pinterest.com/pin/84653667977299238/