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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 November 9th |
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Watermelon Tourmaline from Nigeria: Watermelon tourmaline is one of the most distinctive gems in the world. We have just purchased a small number of exquisite pieces from Nigerian with outstanding color and clarity. These are the finest we've seen this year. |
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Boulder Opal from Australia: Boulder Opal is the second most prized form of opal, after black opal. The name derives from the fact that this opal is found embedded in ironstone boulders. Boulder opal is especially attractive because, like the black opal, it has a dark body tone which adds vibrancy to the play of color. We've just purchased a small lot of excellent pieces, in sizes between 4 and 15 carats. |
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Clinohumite from Tanzania: Clinohumite is a rare mineral. It is a member of the humite group of minerals, which includes humite, clinohumite, chondrodite, and norbergite. Only three sources of gem-quality clinohumite are known: the Pamir Mountains of Tajikistan, the Taymyr region of northern Siberia and the Mahenge plateau of Tanzania. Usually clinohumite is found in sizes under 1 carat but we just have acquired a small number of new pieces from Tajikistan in the 2-3 carat size. |
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Huge Topaz from Brazil: Gem-quality minerals are sometimes found in exceptionally large sizes. We've acquired some natural topaz from Brazil in sizes between 360 and 507 carats. We currently have 4 pieces in a light champagne color with outstanding clarity (all of been graded IF). Collectors take note. |
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Fluorite from Namibia: Fluorite is said to be the most colorful mineral in the world. We've just bought a new stock of some very colorful pieces, including electric greens and blues and some fascinating multicolor pieces. Most of the new pieces are in the 10 to 45 carat size, with a few larger stones, all with excellent clarity. We've recently added some very colorful cabochons as well. |
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Color-Change Diaspore from Turkey: Diaspore, sometimes marketed under the name Zultanite, is a color change gem from Turkey recently introduced to the international market. Under natural or fluorescent light, Diapsore has a kiwi green color, with flashes of yellow. Diaspore displays a champagne color under incandescent lighting, and when exposed to subdued lighting, such as candlelight, has a pinkish hue. We've recently bought a substantial number of new pieces, with a good choice of cuts and sizes. |
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Snowflake Obsidian from Mexico: Obsidian is a naturally occurring volcanic glass. It is formed when felsic lava extruded from a volcano cools without crystal growth.In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern, and this is known in the gem trade as snowflake obsidian. |
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Unheated Sapphire from Tanzania, Madagascar and Thailand: Our unheated sapphires have been very popular and we continue to increase our stock (now over 700 pieces). For the discriminating gem buyer who will accept nothing less than 100% natural, you'll find pink, blue, green, yellow and violet pieces, all guaranteed to be unheated. You may be surprised how affordable these are in the smaller sizes. |
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Seraphinite from Russia: Seraphinite is a trade name for a particular form of clinochlore, a member of the chlorite group. The dark green color of seraphinite is enhanced by a silvery and feathery shimmer caused by mica inclusions. Seraphinite was named for the seraphim, the highest order of angels, because of the feather-like appearance of the chatoyant fibers in the stone. This fascinating gemstone is a new addition our inventory. |
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Top Grade Amethyst from Brazil: We've just acquired some top grade natural amethyst from Brazil in very large sizes, ranging from about 20 to 75 carats. These are all VVS or IF clarity in the richest purple we've seen in amethyst in a long time. If you've been looking for a very special piece of amethyst, you'll want to check these out. |
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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 100 pieces of Namibian demantoid, including some very fine matched pairs. |
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Amber from Poland: Amber is the fossilized hardened resin of the pine tree, Pinus succinifera, formed mainly in the Eocene epoch of the Tertiary period, about 50 million years ago. Amber has been used since prehistoric times for jewelry, amulets and religious objects. The most prized pieces contain inclusions of insects or plants or pyrites. Most amber is found in the Baltic region. We buy amber just a few times a year from our source in Poland and we've just added a new stock. |
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Turquoise from Afghanistan: Turquoise has been known and valued for thousands of years. We've bought some new pieces from Afghanistan, in an interesting variety of blue and blue-green hues. We've also found some of the popular apple-green turquoise that we first carried last year, and some superb matched pairs in blue. |
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Huge Star Rose Quartz from South Africa: We carry many large gemstones, but none as large as these Star Rose Quartz. We've collected 9 pieces over 100 carats, with the largest weighing in at an amazing 974 carats. The star effect in rose quartz is unusual in any size, but quite rare in these very large stones. |
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Axinite from Tanzania: Axinite is a group of brown to violet-brown or reddish brown minerals that sometimes occur in gem quality. Axinite is distinctive for its strong vitreous luster when polished, and its interesting pyro- and piezo-electric properties. Axinite is also popular with mineral collectors due to its unusual crystal structure. We have found a small stock of golden brown Axinite from Tanzania. |
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Spinel from Burma: We buy fine spinel whenever we can find it, and we've recently added some very fine pieces of Burmese spinel. You'll find red, violet, orange, blue and gray in a variety of shapes and sizes, including a good selection of pairs and lots. |
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Blue Sapphire from Madagascar, Ceylon and Tanzania: One of the most classic of all colored gemstones, the blue sapphire is always fashionable. This brilliant and hard-wearing gemstone is perfect for any kind of jewelry. We've added many new stones to our stock of more than 400 blue sapphires, including a number of excellent cornflower blue pieces. |
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Jadeite from Burma: Jade has been known and treasured for more than 7,000 years. But only in 1863 was it discovered that jade is actually not a single mineral. What was traditionally called jade is in fact two separate and distinct minerals: jadeite and nephrite. Jadeite is the rarer and more valuable jade, with the most famous deposits coming from Burma. We have verified samples of our Jadeite at the AIGS gemological lab in Bangkok. You'll find a good range of colors in these, including white, green and lavender. |
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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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Color-Change Garnet from Tanzania and Madagascar: We've recently found some rare Color-Change Garnet from Tanzania with a dramatic color change from greenish-brown to red pink. We have a small number of pieces, mostly eye-clean, in the 1 to 2 carat size. We've added these to our substantial stock of Color-Change Garnet from Madagascar. |
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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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Emerald from Colombia: We have just received a new stock of Colombian emeralds with rich color and good transparency. These are mainly ovals and pears in smaller sizes, ideal for earrings and ring settings. |
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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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