GemSelect Newsletter - August 2009
When most people think of garnet, they think of the common red garnets like almandine and pyrope. But the garnet family is actually remarkably diverse. Garnets can occur in nearly any color, including brown, yellow, orange, red and purple. The green garnets are among the rarest. Recently we've been lucky in finding some excellent specimens.
The garnet group is conventionally divided into 6 species: Pyrope, almandite, spessartite, grossular, andradite and uvarovite. The different garnets share a similar crystal structure, but differ slightly in their chemical compositions. Green garnets are found in both the grossular and andradite types.
Grossular garnets include two green garnets - the yellow-green grossularite and the chrome green tsavorite. The name "grossular" comes from the botanical name of the gooseberry, grossularia. The garnet known as grossularite is typically golden green, yellow-green or olive-green. The rarest grossular garnet is tsavorite, which is a rich green to emerald green garnet variety found only in Kenya and Tanzania. The outstanding green of tsavorite is thought to be due to trace amounts of chromium and/or vanadium.
Tsavorite has become a popular alternative to emerald, though tsavorite is actually the rarer stone. The more common grossularite has its own attractions. Grossularite displays similar brilliance to tsavorite, due to a refractive index that lies midway between spinel and sapphire. The lighter colors of grossularite tend to have better dispersion (fire).
The rarest of all garnets - and one of the rarest of all gemstones - is the green demantoid garnet. Particularly fine examples can command thousands of dollars per carat. Demantoid belongs to the andradite variety of garnet, which is calcium iron silicate with a refractive index of 1.88 to 1.94. The high refractive index makes demantoid the most brilliant of all the garnets; in fact it has a higher refractive index than sapphire and ruby. Demantoid also has remarkable dispersion (fire) that exceeds even that of diamond. Indeed, the very name demantoid means "diamond-like".
Most of the green garnets, especially demantoid, are found only in small sizes. Specimens weighing over one carat are uncommon and stones over 2 carats count as very rare indeed.
Each month we feature a rare and unusual gem from our inventory. This month we feature an unusual black star sapphire from Thailand:
Black star sapphires with gold stars are found in only one location; our home province of Chanthaburi in Thailand. They are also special because they are completely untreated. Most blue star sapphires have been diffusion-treated with rutile to improve the star, but these black star sapphires tend to have a distinct star without any treatment. The 55.93 ct stone shown above is especially rare because it displays a degree of perfection that is rarely seen. Due to its almost round dimensions (22.36 x 20.53 mm) and its well-centred and vivid 6-ray star, this is a true collector's item.
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Happy Gem Hunting!
Your friends at GemSelect
- First Published: August-01-2009
- Last Updated: June-29-2017
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