Gem silica is the tradename for a rare gemstone with a vivid greenish-blue hue. Also known as gem chrysocolla, it is actually a member of the chalcedony family and not, strictly speaking, a variety of chrysocolla at all.
Chalcedony is a type of aggregate quartz. When most people think of quartz, they think of macrocrystalline quartz, which includes amethyst, citrine, ametrine and smoky quartz. Yet many varieties of quartz are cryptocrystalline; they have microscopically (or submicroscopically) small crystals and are usually translucent to opaque, with a waxy, greasy or dull luster. This kind of quartz has fibrous and granular subcategories. The fibrous varieties are known under the general name of chalcedony, while the granular varieties are known as jasper.
Chrysocolla is one of the lesser known gemstones, but is notable because it is one of the few gemstones that contains copper. By chemical composition, chrysocolla is hydrous copper silicate and the copper lends it a distinctive greenish blue color that is similar to turquoise. However, chrysocolla is quite a soft mineral, rating about 2 to 4 on the Mohs hardness scale. It is often found mixed with malachite, azurite, turquoise or quartz.
The reason why rare greenish-blue chalcedony is referred to as gem chrysocolla is that it is colorless chalcedony that has been stained by the same copper salts that give the mineral chrysocolla its distinctive color. High-quality specimens of gem silica are translucent to transparent and highly saturated in color, with excellent color consistency and clarity. They combine the attractive color of chrysocolla with the superior hardness of chalcedony quartz.
Gem silica or gem chrysocolla is very rare, especially high-quality specimens since it forms under unusual conditions when deposits of chrysocolla are visited by silica-laden solutions. Good quality chalcedony cabochons in other colors typically sell for prices in the range of $3 to $5 per carat. However, prices for top grade gem silica can reach as high as $100 per carat. Buyers considering this highly priced material should also consider chrysoprase, a vivid green type of chalcedony that is still affordably priced.
- First Published: December-07-2009
- Last Updated: January-22-2019
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