Sillimanite is an important aluminosilicate mineral and member of the polymorph group that includes andalusite and kyanite, all sharing the same chemical composition but differing in crystal structure. Sillimanite typically forms in long, slender, fibrous crystals. It is a common mineral in high-grade metamorphic rocks and is often associated with other metamorphic minerals.

Sillimanite was named in 1824 after the American chemist and mineralogist Benjamin Silliman (1779–1864), one of the first professors of science in the United States. Silliman was a pioneering figure in American geology and chemistry, and the mineral was named in his honor.

Sillimanite forms under high-temperature, high-pressure conditions typical of regional metamorphism. It is particularly common in metamorphic rocks such as schists and gneisses, where it develops as a result of the metamorphism of clay-rich sediments. Sillimanite is found in high-grade metamorphic terrains around the world with notable deposits in Sri Lanka, India, Myanmar, the USA and Madagascar.

Hardness: 6.5-7.5

Color: White, brown, yellow, green, or gray

Structure: Orthorhombic

Care: Remove before contact sports, weight lifting or activities where it may come into contact with force. As always, it should be stored away from minerals higher on the MOHs scale.

Cleaning: Warm water, mild soap, and soft cloth.

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