Sillimanite Sillimanite is an aluminum silicate, related to both andalusite and kyanite. In fact these three minerals are polymorphs - they share the same chemical composition but different crystal structures.
Sillimanite is named after the American chemist Benjamin Silliman (1779-1864). Silliman was one of the first American professors of science and taught at Yale University.
Sillimanite gems may show a cat's eye and be cut en cabochon, or they may be faceted, transparent gemstones. The colors of sillimanite include colorless, grayish-white, green, bluish and brownish. A fibrous variety of sillimanite is traded as "fibrolite". Some sillimanite gemstones show pleochroism, which means that several colors may be exhibited in a single gemstone depending on the viewing angle.
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