Gemstone Glossary

Sillimanite

Sillimanite occurs in dark brown, brown, pale green or white. It was named after the American chemist and mineralogist, Proffesor Benjamin Silliman (1779-1824).


Sillimanite is found in several locations worldwide including: Brazil, central Europe, Myanmar, Sri Lanka, Czech Republic, India, Italy, Germany and Idaho in the United States. Sillimanite is formed from aluminum silicate (Al2SiO5) and is mostly found as silky, fibrous masses and (rarely) as transparent prismatic crystals with glassy luster. It is sometimes called Fibrolite, alluding to its fibrous massive crystals.

Pure specimens of Sillimanite are not terribly common. Typically it is found scattered within layers of metamorphic rocks which have been put under great pressure and high temperature. This is why Sillimanite is commonly found in volcanic or hot spring areas. According to the way it is scattered within rocks, Sillimanite can only be detected with effort and a good microscope. Best field indicators of Sillimanite are its crystal habit, color, brittleness and hardness if not fibrous.

Sillimanite has a hardness of 7.5 on the Moh’s Scale and a Specific Gravity of approximately 3.2. Its fibrous crystal is quite an interesting mineral form and it is collected as a mineral specimen while the glassy crystal is used as a gemstone and for ornamental carvings. Sillimanite crystals can display a cat’s eye which can be very attractive. The cat’s eye effect is caused by inclusions of fine, slender parallel fibers that reflect light into a single dynamic sliver running along the center of the crystals. When polished as cabochons these stones exhibit a special characteristic that resembles the eye of a cat caught in headlights at night.