Taaffeite (pronounced TAR-fite) is one of the rarer gemstones known, and you'll often it see on lists of the most exotic stones in the world, along with esoteric minerals such as painite, musgravite and grandidierite.
Taaffeite was first discovered in 1945 by the Irish-Austrian gemologist Count Edward Charles Richard Taaffe (1898 - 1967). Taaffe was sorting through a parcel of cut and polished Sri Lankan spinels when he discovered a stone that he knew could not be spinel since it was so obviously doubly refractive (spinel, like garnet and diamond, is singly refractive). This is one of the rare cases where the first sample of a new gem variety was found as a faceted stone.
But apart from being doubly refractive, taaffeite has many similarities to spinel. It is very hard, with a Mohs hardness rating of 8 to 8.5, compared to spinel at 8. Taaffeite has a specific gravity of 3.60-3.62, almost identical to spinel (3.54-3.63); and a refractive index of 1.719-1.730, similar to spinel at 1.712-1.762. By chemical composition taaffeite is magnesium beryllium aluminum oxide (whereas spinel is magnesium aluminum oxide). The most common inclusions in taaffeite are apatite and zircon crystals.
Taaffeite Rough Stone
Taaffeite is known to occur in colorless, violet, red, green and blue. The most common colors are fairly unsaturated mauve and lavender. Deeper reds and purple, colored by traces of chromium and iron, are extremely rare. The largest taaffeite to reach the market was a 33 carat stone sold at auction in Hong Kong in 1999.
Most taaffeite has been misidentified as spinel. But as awareness grows of the differences with spinel, more taaffeite is being discovered. Thus far, Sri Lanka and Tanzania appear to be the only sources for this material. However, because of the geological connection between the deposits in Sri Lanka, Madagascar, and East Africa, gemologists expect eventually to discover taaffeite in Madagascar as well.