Sillimanite is a rare, transparent to opaque gemstone that occurs in colorless, grayish-white, yellow-green, bluish and brownish hues. The luster of sillimanite is vitreous to silky. Sillimanite belongs to the same group of minerals as andalusite and kyanite: they share a similar chemical composition, but have differing crystal structures. Sillimanite is the rarest of them.
Sillimanite is named after the American chemist Benjamin Silliman (1779-1864), the founder of the American Journal of Science. It was first described in 1824 for an occurrence in Chester, Middlesex County, Connecticut, USA. Sillimanite is usually formed at high temperatures and is known to be a useful heat-resistant and chemical corrosion resistant material that is used in iron and steel smelting, and the cement, ceramics and glass industries.
The hardness of sillimanite along with its resistance to heat means that it has good durability. However, due to its rarity, sillimanite is not often seen in jewelry. Sometimes, faceted sillimanite is dyed to imitate precious gemstones such as rubies and emeralds. Sillimanite gems may appear similar to a variety of other stones, such as beryl and moonstone. Like diamond and topaz, sillimanite has perfect cleavage, which means that care should be taken when setting and wearing sillimanite, since a hard blow could cause it to split.
A common variety of sillimanite is known as FIBROLITE, so named because the mineral appears like a bunch of fibres twisted together when viewed in thin section or even by the naked eye. When the fibers of sillimanite are chatoyant and aligned in a certain way, they exhibit a strong and sharp cat's eye effect, which makes it valuable among collectors cut as cabochons rather than faceted gemstones.
Some sillimanite gems possess strong pleochroism, which means that three colors may be exhibited in one gemstone depending on the viewing angle. The secondary colors typically seen in faceted sillimanite specimens are yellow, green and blue. Transparent sillimanite gems are often cut into ovals to preserve the most carat weight.
There are various sources of sillimanite, including India, Burma, Kenya, Sri Lanka, Europe and the USA. However, gem quality sillimanite, especially transparent material is rare. Sillimanite has also been found in New Castle County, Delaware, USA, and in 1977 was named as the state mineral of Delaware, though it was first identified in Connecticut. There are sillimanite findings in other areas, such as South Carolina and Wyoming. Some sillimanite gems from the Mogok region of Burma and the gem gravel of Sri Lanka can appear similar to light blue to colorless sapphire.
