Saturday morning a group of 12 signed in for the two and a half hour drive south to Sycamore Creek. We met Larry Jensen who led our group on a 2 mile dirt road where we all found places to safely pull to the side of the road at a dry wash to search for Red Jasper. Weather was warm and comfortable.
Being a Saturday there were many dirt bikes traveling the road as well as weekend campers to keep us busy. Most of the group canvased the wash area for Jasper and a few of us brought out the 15 pound sledge hammer and steel chisels to assist breaking apart rocks suspected to be hiding good cutting material. I saw members carrying 5 gallon buckets of Red Jasper back to their vehicles so I assume we all found a little something to work with.
Next time when we return I would prefer a weekday trip as there are three locations to collect material and we only saw the first site because of the off-road vehicle traffic.
Gemstones of the mineral corundum [Ref 1] offer a rainbow of colors for the lapidarist and jewelry maker as displayed in Figure 1.
Traditionally, of these, the ruby and blue sapphire, along with diamond and emerald, are considered to be the four-membered family of precious gems. Corundum gemstones, other than the ruby and blue sapphire, are also considered sapphires, having colors ranging from green to pink.
In this blog, I’ll describe the crystallography of corundum, and the physical and optical properties of corundum, including the sources of the colors in its gemstones. I will also present a gallery of ruby and sapphire mineral specimens.
Corundum crystallizes in the Trigonal System, which has three axes in a plane and are arranged at 120 degrees to each other, with an axis perpendicular to the plane, as shown in Figure 2. Of the typical forms of crystals shown in the figure, corundum frequently crystallizes as a hexagonal prism, terminated by the basal pinacoid; as a bipyramid, the hexagonal prism is terminated by a bipyramid; the rhombehedron and the hexagonal prism are terminated by the rhombehedron, and the schalenohedron. Examples of corundum crystals taking these forms are shown in Figures 5-11. Figure 7 shows a diagram of a crystal exhibiting all of these forms except the rhombohedron and schalenohedron. The latter form is shown by the sapphire crystal in Figure 10.
Multiple twinning on the rhombohedral plane with laminar structure with striations on both the basal pinacoid perpendicular to the c-axis and the hexagonal prism or on bipyramid faces, as shown by the terminated bipyrimidal sapphire crystal, shown in Figure 10 [Ref 12]. Corundum is also twinned on the hexagonal prism faces of tabular crystals exhibiting an arrowhead shape, as shown by the sapphire specimen in Figure 11[Ref 13]. The view is at the base of the arrowhead shape and pointing towards the tip. Less frequent twinning in corundum occurs on the basal pinacoid, perpendicular to the long axis of the crystal, as showing repetitive twinning along its length in Figure 12.
The view is at the base of the arrowhead shape and pointing towards the tip. Less frequent twinning in corundum occurs on the basal pinacoid, perpendicular to the long axis of the crystal, as showing repetitive twinning along its length in Figure 12.
The view is at the base of the arrowhead shape and pointing towards the tip. Less frequent twinning in corundum occurs on the basal pinacoid, perpendicular to the long axis of the crystal, as showing repetitive twinning along its length in Figure 12.
The high values of hardness and ultimate strength and its resistance to cleavage, underlie the toughness of corundum gemstones and their wide usage in rings and bracelets, both susceptible to impact while worn. Values of the strength factors of corundum are summarized in TABLE I.
The Refractive Index values of corundum lie in the ranges 1.759-1.772 depending on direction of light polarization. These values are considerably below the value of 2.418 for diamond [Ref 3], and underlies the beauty of corundum gemstones being in their vivid colors and not in brilliance or fire.
The light reflected from the surface, without penetration into gemstones is colorless, as often seen in photographs of gemstones, as in Figure 14.
Light scattering from oriented needle-like crystals of rutile, or to colloidal or other material in oriented tubules is observed in the star sapphire and star ruby as described in another blog on star rubies and sapphires[Ref 4].
SOURCES OF COLOR IN CORUNDUM GEMSTONES
Corundum is aluminum oxide, with the formula Al2O3. Each trivalent aluminum Al3+ ion is surrounded by six oxygen ions, located at the tips of an octahedron in the crystal lattice of corundum, shown in Figure 15. Defects in the forms of ions of metal impurities substituting for the aluminum ion, are responsible for the colors of corundum[Ref 5 ]. The impurity metal ions and the associated colors are summarized in Table I, shown in Figure 4. The divalent and trivalent ions substitute for the aluminum ion in the lattice of the corundum lattice.
COLOR CHANGES IN HEAT TREATED SAPPHIRES
Consideration of the various colors in natural sapphires, having different combinations and concentrations of the ions and ion pairs, before and after their heat treatment, serves to demonstrate their effects on color in corundum gemstones. The results of heat treatments are shown in Figures 16-18.
Some sapphires are heat treated to improve the attractiveness of their colors. Studies were carried out to identify changes in concentrations of ions that led to improvements in the aesthetics of the gem stone. The studies showed two major effects in the brown-toned sapphires and in the optical absorption spectrum of sample rO 4/5, red orange. The red trace of the absorption spectrum shows increased absorption due to the chromium ion, a decreased absorption due to trivalent iron ion pairs contributed from paired divalent and trivalent iron ions and single trivalent iron ions. The heat treatment resulted in an increased number of paired divalent iron ions and tetravalent titanium ions. The lessened absorption by iron ions resulted in smaller contributions to the color of the gemstone in the yellow to orange spectral range. Increased trivalent chromium ion concentration resulted in increased absorption of the blue and yellow spectral range and increased transmission in the red spectral range. Increased absorption in the yellow-orange range, due to increased absorption by paired divalent iron and tetravalent titanium ions resulted in increased transmission in the blue spectral range. The lessened transmission in the yellow-orange and increase transmission in the red and blue color ranges resulted in the cherry-pink color of the gemstone.
GALLERY OF SAPPHIRE AND RUBY SPECIMENS
Many specimens on display are from alluvial deposits where erosion of the edges and faces arose from wear against surrounding gravel and sand.
In this blog beautiful art objects, carved and sculpted in crystals of the beryl gemstones, are presented. In some, the shape of the gemstone inspired the shape of the object. In others, the color of the object is enhanced by decorating with differently-colored gems. Ancient to modern art objects are presented.
The beautiful blue hues of aquamarine, the rich yellows of heliodor, the deep green of emerald, the subtle pinks of morganite, the deep blue of the maxixe beryl, and the rich color of the red beryl (bixbite) span the rainbow, as shown in Figures 1 and 2. The beauty of these gemstones underlies their use by artisans in the crafting of jewelry and art objects, and it’s importance to collectors.
In this blog, I’ll describe Beryl’s crystal structure and the structural defects and chemical impurities which are the sources of color and fluorescence in beryl. I will also present a summary of sources for beryl gemstones and a photo gallery of mineral specimens from around the world, which I found to be not only aesthetic, but representative of the crystal habits of beryl and demonstrated the color variations and fluorescence in beryl gems.
COLORS OF BERYL
Beryl is a beryllium aluminum silicate with the formula BE3Al2(Si6O18)[Ref 2].
Substitution of other metals as impurities for the intrinsic beryllium and aluminum in the crystal lattice of beryl, are the causes of color in beryl. The positions of beryllium, silicon and aluminum ions in the crystal lattice of beryl are shown in Figure 3 [Ref 3]. The divalent beryllium ion (Be2+) is in coordination with four neighboring oxygen atoms, (tetrahedral coordination), and the aluminum ion (Al3+)in coordination with eight oxygen ions, (octahedral coordination). Twelve silicate tetrahedrons are arranged in circular patterns forming rings. The vertical stacking of the 2-D lattice shown in Figures 3.a results in a stacked ring arrangement which forms channels, as shown in Figure 3.b. Water molecules and alkali metal ions, like the monovalent sodium ion Na1+ are located in the channels [Ref 4].
The impurity metal ions and free radical anion responsible for the colors of beryl gemstones are presented in Table I. The process resulting in the absorption of light is electron transfer from a neighboring oxygen to the metal ion. The absorption of light, in regions of the visible light spectrum, defines other wavelengths of the spectrum where the absorption is small and is transmitted or reflected. As an example, the absorption spectrum of heliodor, the golden beryl, taken in polarized light and shown in Figure 4, show a wide absorption peak at ~810 nm in the near infrared part of the spectrum and the ascending flank of a rising peak at a wavelength less than 300 nm in the ultraviolet. Comparison of the wavelength region with low absorption of light, with the colors of the visible spectrum displayed in Figure 5, show the transmitted or reflected light to fall in the yellow-orange range of the spectrum.
All metal impurity ions listed in Table I fluoresce upon UV light absorption, and serve as activators of fluorescence in beryl. Colors of fluorescence which have been observed in beryl with excitation by a UV lamp are shown in Figure 6 [Ref 6]. Emeralds also exhibit red fluorescence as shown in Figure 4, but require the intensity of laser light for visible fluorescence to be observed. [Ref 7, Ref 8]. The reason for the requirement of intense light for activation stems from the effects of the surrounding lattice about the chromium ion Cr2+ which increases the energy density, (intensity), of light required to excite the electron transitions responsible for fluorescence. A UV lamp is sufficient to excite the chromium ion in ruby because the effect of the lattice on the electrons of the ion is less in ruby.
GALLERY OF FLUORESCENT BERYL SPECIMENS
SOURCES OF BERYL GEMS
Sources of gem varieties of beryl extend around the world and are summarized in Tables II and III. Many sources of gem-beryl crystals are a cavity in a
Pegmatite [Ref 5] located within granite [Ref 6]. Others are located in rocks associated with pegmatites. Red beryl is found in the extrusive igneous rock rhyolite. Differently, the emeralds are found in veins of calcite in shale in the mines at Muzo, Colombia. Worldwide locations are summarized in Tables II and III.
GALLERY OF GEM BERYL
The specimens shown in Figures 6-18 were selected to demonstrate the variations in the forms of gem beryl crystal and the variations in the characteristic colors of gem beryl. The crystal forms range from a hexagonal prism terminated by a flat face as in Figures 6-11, to prisms terminated by one or more families or pyramidal faces as in Figures 12-18, and to crystals exhibiting higher order prismatic faces as in Figure 11.
Figure 13. Morganite on feldspar, Chapa Dara District, Konar Province, Afghanistan [Ref 22]. Six narrow pyramidal faces truncate the flat termination of the tabular crystal. The peach tone of the crystals stems from the presence of the ferric ion Fe3+ which imparts the golden color of heliodor to the pink coloration of morganite as seen in Figure18.
The emerald is the chromium-containing variety of the mineral beryl [Ref 1]. Its’ beautiful, vibrant, deep green color has made it a great favorite among those that create beautiful jewelry and art works from it, gem & mineral collectors and those who wear it. Witness the Hooker Emerald, set with diamonds, in a brooch, seen in Figure 1 [Ref 2]. Mined in Colombia, polished in Europe, and sold in the 17th century to the ruling family of the Ottoman Empire, it was then auctioned in 1911 to Tiffany & Company and ultimately set in a brooch. It finally sold to Janet Annenberg Hooker who donated the brooch to the Smithsonian Institution.
A HISTORY OF EMERALDS THROUGH JEWELRY AND ART WORKS
In this blog I’ll describe the ancient history of the emerald as a gemstone, beginning
with their earliest-known written description in the Egyptian Papyrus Prisse during the twelfth dynasty of Egypt, from 1991-1802 BC [Ref 4], and arbitrarily ending with jewelry of the Georgian Era[Ref 5] in Europe and the end of the of the Ottoman Empire[Ref 6] in the East at the end of World War I.
Ancient History of Emeralds As A Gemstone
The name Emerald, evolved from esmeralde in 16th century Latin.
Emerald is an ancient gemstone. The first known written word about emeralds appeared in the Papyrus Prisse [Ref4] dated in the twelfth dynasty of Egypt (1991-1802 BC [Ref 7] and in which was written “But good words are more difficult to find than the emerald”. Emeralds were mined in Egypt, with the earliest-known mines operating from at least 300BC into the 1700s. Emeralds, highly prized by Cleopatra, probably came from these mines.
The Roman scholar Pliny, was first to suggest that emerald was a member of the larger family of the mineral beryl. Only in the 19th century did science recognize this relationship.
The emerald mines of ancient Egypt, known as the mines of Cleopatra, were located approximately 180 miles (300 KM) SSE of present-day Cairo, in a coastal region along the Red Sea, as shown in the map in Reference 8. Their history and locations are further discussed in Reference 9. The mines were the major source of emeralds for both Europe and the East, until emeralds of finer quality and greater abundance were found in Colombia in the 1520’s[Ref 8, Ref 9].
History of Emeralds in the Americas
Emeralds were prized by the Aztec, Olmec, Mayan, and Incan cultures of Middle and South America. They have been mined since early times, in areas of what is now Colombia, as evidenced by the Olmec carving “Emerald Man”, shown in Figure 2, which dates from approximately 500 BC to 250 AD [Ref 10].
Upon arriving in Mexico and Peru in the 16th Century, conquistadors discovered the natives of these lands possessed large, beautiful emeralds in the form of carved jewelry and artworks. Five gemstones brought by Cortez to Spain were cut into shapes of flowers, fishes and other natural objects. Emerald carving was an ancient art in the cultures of central and south America.
Emeralds were prized as an ornamental gemstone and held to be sacred. They were offered to their Gods and buried with their dead. However, the Conquistadors failed in their search for the sources of emeralds in Mexico and Peru. They desecrated sacred places in their searches, amassing a large treasure trove. Jose d’Costa reported that the ship on which he sailed from Peru to Spain in 1587 carried two cases, each with at least a hundredweight of emeralds.
The conquistadors failure in their search for the sources of the emeralds in Mexico and Peru was because the emeralds were mined only in the heartlands of the Chibcha Indians at Chivor, and the Muzo Indians at Muzo, in what is now Colombia. (Mines in these places still produce emeralds). Trade routes for emeralds extended to Mexico in the north and to Bolivia in the south. Ther’s a good chance that the “Emerald Man” was carved from a stone from one of these mines.
Ancient Emerald Jewelry of Europe and the East
Through trade routes, emeralds from the mines in Egypt were disseminated and reached artisans in ancient Europe [Ref 11], and in the Middle East, as evidenced by their creations of jewelry and art works. The craftsmanship and beauty of their works are exemplified in Figures 2-34.
For slide shows of the galleries please make two, one for ancient Europe and one for the Eastern Mughal and Ottoman jewelery and separate references accordingly. Thanks.
A Gallery of Emerald Jewelry of Ancient Europe
The jewelry in this gallery spans the time period between the 2nd Century BC to 1780 AD in the Georgian Era.
The hull, mast, and sail are of emerald. The caravel is a small ship designed for maneuverability [Ref 21].
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.
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.
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
~ .5-.7 microns
~ .5-1.4 microns
8 A, B, C
~ 1-1.5 microns
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.
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).
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.
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.
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].
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.
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.
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.
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.
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].
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.
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].
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].
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.
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].
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].
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.
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.
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].
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).
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.
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.
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.
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.