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 July 16th |
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Pink Tourmaline from Mozambique: Pink is the most popular color in tourmaline and we have just expanded our stock with some outstanding new pieces. The shades range all the way from soft pink to hot pink and purple pink. We have more than 350 pieces in all, in a wide range of sizes and shapes. We buy pink tourmaline whenever we can find it, but rarely find this many fine pieces. |
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Chrysoberyl Cat's Eye: The most famous and valuable cat's eye gemstone is chrysoberyl cat's eye. In fact when the term cat's eye is used alone in the gem trade, it always refers to chrysoberyl cat's eye. All other types of cat's eye gems require an additional varietal designation, such as cat's eye apatite. Chrysoberyl cat's eye is so highly regarded due to its excellent hardness (8.5 on the Mohs scale), superb luster and remarkably sharp cat's eye effect. We have just found some excellent pieces at an unusually good price. |
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Hessonite Garnet from Madagascar: Hessonite belongs to the grossularite garnet group, which also includes tsavorite garnet. While most grossularite garnet is green, hessonite typically has a orange-brown color which has earned it the nickname cinnamon stone. We rarely find hessonite in the market, but we've just bought an excellent selection of matched pairs in a variety of shapes. |
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Star Diopside from India: Diopside is a calcium magnesium silicate with a hardness of 5-6 on the Mohs scale. Diopside is best known for the vivid green chrome diopside, but the black diopside exhbiiting asterism or the star effect is rare and unusual. We've acquired a small number of pieces from India, in very impressive sizes (upt to 48 carats). Collectors take note, we hardly ever see these. |
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Carnelian from India: Carnelian is a brownish red to orange variety of chalcedony quartz, colored by trace amounts of iron. Carnelian is one of the oldest known gemstones, being widely used in ancient Rome to make signet or seal rings. Most of the carnelian you'll find in the market is agate which has been dyed and heated, but we have found a very fine parcel of hard-to-find untreated natural carnelian, ideal for gem collectors. |
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Rhodochrosite from Argentina: Rhodochrosite is an interesting rose-colored gemstone that was only introduced to the market around 1940. Though it is not particularly hard (about the same as fluorite) it has become popular as a decorative material due to its attractive soft color and its intriguing variegated markings. We've found a nice parcel of Argentine rhodochrosite and have a good selection of affordable cabochons in sizes up to 12 carats. |
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Morganite from Afghanistan: The pink form of beryl was named Morganite, after the American banker and collector J.P. Morgan. A soft pink to violet, Morganite belongs to the same family as emerald and aquamarine. Like the other beryls, Morganite is quite hard (7.5 to 8 on the Mohs scale). We've just acquired a good range of high quality pieces, mostly in the 1.5 to 2.0 carat size. These are clean and well-cut stones with the delicate pink hue for which morganite is famous. |
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Chrysoprase from Tanzania: Chrysoprase is considered the rarest and most valuable stone in the chalcedony quartz group. That's quite a distinction considering that the chalcedony species includes agate, chalcedony, onyx, carnelian, bloodstone and jasper. But chrysoprase, it is fair to say, is also the least known of all the chalcedony varieties. Chrysoprase is a vivid apple-green to deep green gem, colored by trace amounts of nickel, and makes stunning jewelry. |
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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 in larger sizes. You'll find reds, violets, blues and grays in a variety of shapes, including a number of hard-to-find cushion cuts. Sizes range from 1.5 carats all the way up to 5 carats. |
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Ruby-Zoisite from Tanzania: Ruby-Zoisite, also know as anyolite, is the natural combination of ruby and zoisite crystals in a single specimen. Zoisite, the same mineral as tanzanite, provides an earthy green and black color while ruby lends exciting pink and red highlights. Ruby-zoisite is usually cut en cabochon or carved. We've recently purchased some excellent cabs, but also some unusual and rarely seen faceted peices. |
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Dendritic Agate from India: Dendritic agate is a whitish-gray or colorless chalcedony with fern-like inclusions known as dendrites. The inclusions look like plant material, but they are actually iron or manganese. We've found a small number of fascinating pieces from India. |
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Yellow Sapphire from Thailand: We've just acquired some brilliant yellow golden and yellow orange sapphires from Thailand, mainly in sizes from 1 to 3 carats, with a few larger pieces as well. You'll find a nice range of shapes, with many pieces VS or VVS clarity. |
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Ruby Cabochons from Madagascar: Recently we've found some excellent ruby cabochons from Madagascar in the 3 to 14 carat size. These are pure red and some are clean enough to be graded translucent. These cabs make wonderful jewelry and the price is very attractive as well. |
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Grossularite Garnet from Kenya: Grossularite (or grossular) garnet is a calcium-aluminium garnet. The name grossular is derived from the botanical name for the gooseberry, grossularia. Grossularite garnet combines excellent brilliance and hardness. We've recently acquired some very fine pieces from Kenya, mainly in the 1 to 2 carat size. |
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Fire Agate from Mexico: Agate is a type of chalcedony quartz that forms in concentric layers in a remarkable variety of colors and textures. Fire agate is an opaque, limonite-bearing chalcedony with iridescence which is created by diffraction of light by the layered structure. We've found some excellent pieces from Mexico, in sizes up to around 10 carats. |
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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 color. We have recently added some fine matched pairs to our extensive stock of this popular gem. |
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Sunstone from India: Sunstone is a type of plagioclase feldspar that exhibits a spangled appearance, due to reflections of red haematite. We've recently acquired some interresting sunstone cabochons from India, in a good range of sizes from 5 to 50 carats. |
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Labradorite from Madagascar: Labradorite is a member of the plagioclase feldspar group, which also includes andesine and sunstone. Labradorite displays an iridescence in lustrous metallic tints, often blue and green. It is mainly caused by interference of light from lattice distortion. We've found some very interesting labradorite cabochons in sizes up to 10 carats. |
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Scapolite from Namibia: Scapolite is a sodium calcium aluminum silicate with a hardness of 5.5 to 6 on the Mohs scale. It is named from the Greek for "stick," since its tetragonal crystals grow in columns. As a gemstone scapolite is not well known, but it can be a very attractive stone. Its color, which is usually a virbrant yellow to orange, pink or violet, is its best feature. We've found some excellent pieces in brilliant yellow and gold, in szies from 3 to 6 carats. |
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Cat's Eye Apatite from Africa: Cat's eye apatite can often be confused with the more expensive chrysoberyl cat's eye, but apatite is a softer stone (5 on the Mohs scale) that is best used for pendants and earrings. It does exhibit a very striking cat's eye effect, however, and is sought after by collectors. We've recently bought some first rate pieces in both green and honey, in sizes from 2 to 6 carats. |
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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 180 pieces in stock, in mandarin and red-orange, in both cabochon and faceted. The range of sizes and quality grades is extensive. We've just purchased a number of high grade pieces with superb brilliance, at some of the best prices we've seen in spessartite in the past year. |
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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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