We usually think of star sapphires and rubies as stones in the few carat range, not in hundreds and thousands of carats. But some are an amazing size. At the top, are the World’s Largest which are truly awesome. In this blog I’ll describe the worlds largest blue star sapphire, star ruby, and black star sapphire, as well as gems ranking near their size but just below the record. I’ll also include a short history of each gem.
Worlds Largest Blue Star Sapphire: The Star of Adam [Ref 1, Ref 2]
The world’s largest star sapphire weighs an amazing 1,4O4 carats, and when held, largely occupies the palm of a hand, as shown in Figure 1.
The stone was found in the fall of 2015 at mine in the famed alluvial gem deposits near Ratnapura, Sri Lanka [Ref 1 Ref 2]. At first sight, the owner estimated the value of the gem at $175 million. As of January 2016, the owner was pondering whether to auction the gem or to display it. A diligent trip over the internet disclosed no further information about any attempt to auction the gem. However, the gem may have been sold in a private transaction.
The Black Star of Queensland, as shown in Figure 2, weighs 733 carats, and was the world’s largest sapphire until being displaced by the Star of Adam. This gem is also seen to fill the palm of a hand but does not have the cabochon height of the Star of Adam.
Reportedly, the rough stone was found by a twelve-year old boy, Roy Spencer, in the mid-1930s, in the Reward Claim near Anakie, Queensland, Australia [Ref 3]. The boy’s father, Harry Spencer, assumed it was merely a black crystal and the family used it as a doorstop for over a decade. A second look disclosed the gem.
The stone was sold by Spencer in 1947 to the jeweler Harry Kazanjian for $18,000AU which funded a new house for the family. The subsequent history of the stone has been shared by owners and institutions. The gem was loaned to the Natural History Museum of the Smithsonian Institution in 1969. In 1971 it was seen around Cher’s on television show. To fulfill a childhood dream, the artist and jeweler, Jack Armstrong, and his wealthy girlfriend, Gabrielle Grohe, convinced the Kazanjian family to sell the gem in 2003 [Ref 4]. In 2010 [Ref 5], the pair squabbled over the stone and Armstrong agreed to pay $500,000 for Grohe’s share, but failed to pay, and due to a judge’s ruling he lost all right to the gemstone.
World’ Largest Star Ruby: the Appalachian Ruby Star [Ref 7]
The Appalachian Ruby Star weighs 139.43 carats and barely edges out the Rosser Reeves Star ruby, that weighs in at 138.72 carats, as shown in Figure 3. The Appalachian Star ruby was cut from a rough ruby, which also yielded three additional stones. The weight of the group of four star rubies became known as the Mountain Star Ruby Collection and is shown in Figure 4. The aggregate weight of the rubies totals 342 carats.
The rough ruby was found in 1990 by Wayne Messer, a fishing guide in Western North Carolina. He had noted traces of corundum in a stream bed and traced the alluvial stones back to their source. Upon digging some eight feet, found the rough ruby. The quartet of star rubies was cut by Sam Fore from the rough stone which weighed 377 carats.
The Appalachian Star ruby was exhibited in 1992 at the Natural history Museum in London, drawing an estimated 150,000 people. Several attempts were made over the years to sell the collection, appraised at a value close to $100 million. Only recently, following the death of Messer, was the collection offered for sale.
At a weight of 138.7 carats, the Rosser Reeve Ruby is the world’s second heaviest star ruby, and was found in Sri Lanka. The gem is named after Rosser Reeves a pioneer in the advertising industry. Rosser donated the gem to the Smithsonian Institute in 1965. Despite his attractive tale of buying the gem at an auction in Istanbul, he actually bought the gem from Robert C. Nelson Jr. At purchase, the stone weighed just over 140 carats, but was scratched and so was re-polished, which also helped to re-center the star on the Cabochon. Fortunately for museum goers, this beautiful gem still can be seen at the Smithsonian institute. As a note, the Wikipedia article used as the reference for this segment of the blog was written by Brendan Reeves, great grandson of Rosser.
This was the clubs second field trip for October with the intention of collecting Luna Blue Agate in New Mexico.
A new friend from the Prescott club was to meet us Saturday afternoon in Alpine to lead us to Reserve , New Mexico to collect Agate on Sunday October 27th.
Thirteen members arrived in Alpine, Arizona around noon at the motel and from there we headed east on the 180 towards Luna N.M. We made several stops south of Luna at mile marker 10 and then at mile marker 4.
Good material was collected at MM4 in the road cuts, some Luna Blue was found and Druzy. Not a real productive site but still a good start for what awaited in Reserve on Sunday.
After breakfast Sunday morning we followed Allen Valley to Reserve N.M. where Allen had made arrangements to cross over private property onto State Land which was about a 3 mile drive uphill on uneven dirt road through three closed gates to a plateau in a forested setting. A vast expanse of forest land was covered in Banded Agate with Druzy. It was an overwhelming amount of material. It was hard not to pick it all up, but we had to be selective when we realized how much there actually was.
We spent the day there trying several locations and coming away very happy.
Sunday evening at dinner Allen made arrangements with a local from Alpine to lead us back into N.M. to several locations that turned out to be not so productive. It is my feeling our local guide did not want to take us to any secret spots thinking we would come back at some point and clean them out, can’t blame her for that, but she shouldn’t have offered to waste a whole day driving around. Beautiful country but we wanted rocks too. All in all this was a successful trip.
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.
Four members of the club (Linda and Marty Dougherty and John and Beth Duggan) made it down to Quartzite, Arizona for the annual Pow Wow. This show has been held in January for over 50 years. There were over two thousand vendors to visit in the area so there was lots to see and buy.
For the seventh year in a row Linda and Marty attended the annual roadside Clean up in Quartzsite, Arizona sponsored by the Bureau of Land Management and American Lands Access Association. There were club members of other American Federations, Northwest, California, Mid-west and Rocky Mountain, which the Coconino Club is a member.
Twelve of them plus the cameraman drove 10 miles East of Quartzsite to the Gold Nugget Rd. turn-off and spent a couple hours collecting discarded trash which is apparent in the attached photograph. Not sure how much this years weighed, but in previous years in this area we collected a couple tons of trash, some being really gross!!!!
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.