In our newsletter this month:
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, from brown to yellow to orange, red and purple. But the green garnets are among the rarest. Recently we've been lucky to find some excellent specimens in the market.
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 species.
The grossular species includes 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 a greenish-golden to yellow-green or olive-green. The rarest grossular is tsavorite, a rich green to emerald green garnet 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. But 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. But the lighter colors of grossularite tend to have better dispersion or fire.
The rarest of all garnets -- and one of the rarest of all gemstones -- is the green demantoid garnet. Particularly fine examples can command prices of thousands of dollars per carat. Demantoid belongs to the andradite variety of garnet, which is a calcium iron silicate with a refractive index of 1.88-1.94. 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 or fire that exceeds even that of diamond. Indeed, the very name demantoid means "diamond-like luster."
Most of the green garnets, especially demantoid, are found only in smaller sizes. Specimens over one carat are uncommon and stones over 2 carats count as very rare indeed.
Rare and Unusual Gems
The black star sapphires with the 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 treatment with rutile to improve the star, but these black star sapphires tend to have a distinct star in the absence of any treatment. The 55.93 ct stone shown above is especially rare because it displays a degree of perfection that is rarely seen. From it's nearly round dimensions ( 22.36 x 20.53 mm) to 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