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January 2008
 In our newsletter this month:
Zircon and Zirconology
New and Interesting Gemstones
Gemstones Worth Knowing
Customer Questions
Zircon and Zirconology
You'll often see ad copy for tanzanite touting it as the gemstone that is "585 million years old." It's not clear exactly why that is a selling point for the gemstone buyer, and in fact most minerals are hundreds of millions of years old. But if you're interested in a truly ancient mineral, look at zircon. The oldest known zircon samples, from Western Australia, are more than 4.4 billion years old.
 Zircon is actually the oldest known mineral on earth. Considering that the earth formed 4.56 billion years ago, the oldest zircons represent the earliest record of the earliest crust of the earth. Zircon was the first crystal to form in molten granite as it cooled to form rock. The ancient granitic rock has long eroded away; the only record that is left is tiny grains of zircon. This means that zircon is in fact the oldest thing on earth; the oldest samples are even older than the moon, which formed about 4 billion years ago.
 Zircon is of such interest to geoscientists that it has spawned a discipline of "zirconology." Scientists have discovered that zircon contains trace elements of uranium and thorium, elements with known half-lives. Since uranium decays to lead at a known rate, radiometric dating can be used to estimate the age of zircon. Scientists have also analyzed zircon crystals for oxygen isotopes and have found the telltale signature of rocks that have been touched by water: an elevated ratio of oxygen-18 to oxygen-16. This has led to a rethinking of the origins of life on earth. If there was water 4.4 billion years ago, then life may have existed on earth much earlier than previously thought. On previous theories, the first cells with a nucleus appeared about 1.5 billion years ago.
Zircon is thus a tremendously important mineral scientifically. But for gemologists zircon is also fascinating. It is a remarkably dense mineral, about 50% denser than diamond. That means if you have a diamond and a zircon of similar size, if the diamond weighs 1 carat, the zircon will weigh about 1.5 carats. So be careful to check the dimensions of any zircon you buy or the stone may be smaller than you expected.
 Zircon also has very marked birefringence. Birefringence is a measurement for gemstones which are doubly refractive; that is, gems which have two different refractive indices. When a beam of light enters a doubly refractive gem, it is split into two beams, each travelling at a different speed and on a different path through the crystal. Birefringence is a measurement of the difference between the two refractive indices in gems. Zircon has such high birefringence that a zircon may appear fuzzy or display 'facet-doubling' when you look down through the table. It requires special cutting skill to minimize these effects.
In addition to its notable birefringence, zircon has a high refractive index (1.810 - 2.024), approaching that of diamond (2.417 - 2.419), as well as high dispersion, the tendency to split white light into the spectral colors. What all this means is that zircon is an exceptionally brilliant gem. It's not surprising that in the days before diamond simulants, white zircon was often used as a substitute for diamond. But don't confuse zircon with cubic zirconium, a well known diamond simulant. Cubic zirconium is a recent invention of man; zircon is the oldest natural thing on our planet. It's a stone that every gem collector should have in his or her collection.
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| New and Interesting Gems -- updated March 15th |
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Variscite from the United States: Variscite is a relatively rare phosphate mineral and high quality specimens are used as gemstones and for carvings. Its name derives from Variscia, the historical name of Vogtland in Germany, where it was first found in 1837. At one time it also went under the name Utahlite, since important deposits have also been found in Utah, USA. We've found some excellent cabochons in various shades of green, mainly in the 20 carat size. |
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Goshenite from Russia: The colorless precious beryl is known as goshenite. It is named after the small town of Goshen in western Massachusetts where it was first described. Goshenite was once used for manufacturing eyeglasses and lenses owing to its excellent transparency. Goshenite belongs to the same family as emerald, aquamarine and morganite. We've found a small number of high quality pieces from Russia, in sizes ranging from 3 to 5 carats. |
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Dumortierite Quartz from Namibia: Dumortierite Quartz is an ususual quartz that is integrown with the mineral dumortierite. The inclusions of dumortierite give it a deep blue color that is unique in the world of quartz. The color has similarities to lapis lazuli and sodalite, but with the superior hardness of quartz. We have a nice range of cabochons from 7 to 35 carats. |
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Mali Garnet from Mali: Mali Garnet is a rare garnet that is a mixture of grossular and andradite garnets. The name derives from the West African country of Mali where this garnet was first discovered in 1994. Mali is still the only source for this unusual garnet. Though Mali Garnet is predominantly grossular by composition, the presence of andradite is responsible for its superb dispersion or fire, reminiscent of the most famous andradite garnet, demantoid. |
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Rubellite Tourmaline from Mozambique: Rubellite, the intense ruby-red color of tourmaline, is one of the rarest tourmaline colors. We rarely find good specimens in the market, so we're glad to announce that we've just added more than a dozen new rubellite pieces to our inventory, mainly in the 1 to 3 carat size. |
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Sillimanite Cat's Eye from Madagascar: Sillimanite is an aluminum silicate, related to both andalusite and kyanite. In fact these three minerals are polymorphs -- they share the same chemical composition but different crystal structures. Sillimanite is named after the American chemist Benjamin Silliman (1779-1864). Silliman was one of the first American professors of science at Yale University. We've bought some interesting sillimanite cabochons with an exceptionally vivid cat's eye. |
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Sugilite from Australia: Sugilite is a rare mineral named after the Japanese geologist, Ken-ichi Sugi, who discovered it in 1944. It is rarely found as crystals, usually being massive in form. It has a distinctive purple colour and is usually opaque to translucent. Sugilite has sometimes been called purple turquoise although there is no real connection between turquoise and sugilite, except both are colorful cabochons that are very attractive when set in silver. We've just bought a new stock of some top-grade sugilite. |
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Amazonite from Brazil: Amazonite is a gemstone variety of green microcline, a feldspar mineral. It is named after the Amazon river in Brazil, though surprisingly no deposits have been found in that location. The lively green or blue-green color is reminiscient of jade. We've just acquired a parcel of interesting cabochons in a vivid blue green, including some matching pairs. |
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Color-Change Garnet from Tanzania and Madagascar: The rare color-change garnet from east Africa and Madagascar has been very popular, and we have been buying all the high quality pieces we can find. We have just added several dozen new pieces from Madagascar. These have a striking color change from golden to orange or pink. |
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Charoite from Russia: Charoite is a new gem on the market, first appearing in 1978. It is found only in one location in Siberia, Russia. The color and patterns are quite unique, with swirling shapes of lavender and violet. It is sometimes described as having the appearance of a purple marble. We've just purchased a new lot of charoite cabs in a deep violet, in sizes up to 9 carats. |
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Emerald from Colombia: We have just received a new stock of Colombian emeralds with rich color and good transparency. These are mainly ovals, pears and emerald cuts in smaller sizes, ideal for earrings and ring settings. |
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Cat's Eye Scapolite from Madagascar: Scapolite displaying a cat's eye effect are rare and highly regarded in the gem trade, since they tend to have exceptionally sharp eyes. Just keep in mind that these are slightly soft stones (5.5 to 6 on the Mohs hardness scale) and are not suitable for rings. But they make terrific earrings and pendants. |
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Blue Sapphire Pairs from Madagascar: We have been buying some outstanding blue sapphires from Madagascar recently, and we've put together several dozen matching pairs. These are mainly rounds and ovals, with a few square-cut as well. A classic stone for earrings and perfect for ring and pendant designs. |
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Unheated Yellow Sapphire from Tanzania: Completely untreated yellow sapphire is rare and difficult to find, not to mention expensive. We have been finding some very good unheated yellow sapphire from Tanzania, mostly in sub-carat sizes. But we did find 5 very good pieces between 1.1 and 1.5 carats. These are all ovals with VVS clarity, at an excellent price for unheated material. |
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Large Tourmaline from Mozambique: We have one of the largest inventories of tourmaline you'll anywhere, and we have just added many large pieces to our collection. You'll find pink, red, orange and green, as well as an amazing variety of multicolor and bi-color pieces. We also have a good stock in watermelon crystal slices. |
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Demantoid Garnet from Namibia: Demantoid is the rarest and most valuable of the garnets, and is one of the most difficult to find of all colored gems. It is one of the few colored gemstones with a luster and brilliance similar to diamond. We have built up a stock of more than 150 pieces of Namibian demantoid, including some very fine matched pairs. |
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Spinel from Burma and Tanzania: We buy fine spinel whenever we can find it, and we've recently added some very fine pieces to our extensive stock. You'll find red, violet, orange, blue and gray in a variety of shapes and sizes. Notable among these acquisitions are some larger pieces in the 2 to 4 carat size, and a number of hard-to-find matching pairs. |
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Apatite from Madagascar: Apaptite from Madagascar is famous for a vivid blue-green that is reminiscent of paraiba tourmaline. We have just acquired a range of excellent new pieces. Most are in the 1 to 2 carat size, but we've also found a few rare stones in the 4 to 5 carat size. We also have stock in the popular green apatite, known as the "asparagus stone." |
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Sphene from Sri Lanka: We have added some excellent new pieces to our stock of this rare gemstone famous for its remarkable dispersion or fire. These are bright golden and green pieces in interesting shapes and cuts, including a number of rounds in smaller sizes. Sphene tends to have inclusions, but we've found a number of eye-clean pieces. |
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Chrome Diopside from Russia: Chrome diopside is one of the few gemstones colored by chromium. Its rich green is unique in the gems world, rivalled only by the best tsavorite garnet and chrome tourmaline. We have recently purchased some very fine matched pairs for earrings, as well as a number of very affordable pieces in smaller pieces. |
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Spessartite Garnet from Mozambique: We have added a large number of new pieces to our extensive inventory of this very popular gem. We now have over 500 pieces in stock, in mandarin and red-orange, in both cabochon and faceted. The range of sizes and quality grades is extensive. Our most recent additions are in the best mandarin orange color, mainly in smaller sizes. |
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Gemstones Worth Knowing
Each month we focus on one of the lesser known gemstones. This month's featured stone is Chrome Diopside.
Until recently, fine chromium-bearing green gemstones were retricted to emerald, tsavorite garnet and chrome tourmaline. All were very expensive. In 1988 there was a major find in Russia of chrome diopside, a chromium-rich variety of calcium magnesium silicate. The saturated green of diopside is easily the rival of tsavorite and chrome tourmaline, but the price is substantially lower. The only limitation is that chrome diopside is a softer stone (5 to 6 on the Mohns scale), and it's not suitable for everyday rings. But it is very well suited to earrings and pendants and looks stunning when set in gold.
For more information see our Chrome Diopside information page. and our recent feature article on diopside.
Customer Questions
Every month we answer questions of general interest from our customers. Please feel free to send your questions to help@gemselect.com, with "Newsletter question" in the Subject line. |
| Q: |
As I understand it, both ruby and emerald are colored by chromium. How can chromium produce both red and green colors? I would really like to know, so thanks for your answer. BB, Canada |
| A: |
Both corundum (ruby and sapphire) and beryl (emerald) are colorless in their pure form. Like many gemstones, they are colored by impurities. In the case of both ruby and emerald, it is chromium that produces the color. In fact in both cases aluminum atoms are replaced by chromium. The difference in color is a result of differences in the bond strengths that affect the absorption and transmission of light. In ruby, violet light is strongly absorbed along with green-yellow light. Blue light and red light are transmitted. Transmission of red, however, is much stronger than that of blue, so the ruby looks red. In the case of emerald, violet light is still absorbed but the green-yellow absorption band of ruby shifts to absorption of yellow-red light in emerald. This diminishes the transmission of red light so that the transmission of blue-green light (corresponding to the weak transmission blue light in ruby) is now strongest and determines the color of the emerald. This variation is even more interesting in the case of chromium-rich chrysoberyl (alexandrite), which has bond strengths between ruby and emerald. Alexandrite will transmit blue-green light in daylight and red under incandescent light. |
| Q: |
My friend has a diamond tester which he says can tell real diamonds from fake ones. Do these really work? FG, USA |
| A: |
Many of these diamond testers do work, but it's important to understand how they work and what they are testing. The original diamond testers performed a thermal conductivity test, and could distinguish genuine diamond from simulants such as cubic zirconium. But then moissante, a new diamond simulant, came on the market, and it had thermal conductivity values very close to diamond, and first generation diamond testers could not identify it. A second generation tester that also tests for electrical conductivity can distinguish moissanite, since moissanite conducts electricty and genuine diamond does not. Just remember that synthetic diamonds are genuine diamonds and will be identified as such by these simple testers, since their thermal and electrical conductivity is the same as natural diamond.
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| A final note -- If you send us email, please be assured that we answer all our email very promptly, 6 days a week. But we sometimes have problems with spam filters on the receiving end, so please adjust the settings on your mail client so you can receive mail from help@gemselect.com.
All the best for 2008!
Your friends at GemSelect |
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