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Sillimanite is a metamorphic mineral, primarily composed of aluminum silicate (Al2SiO5), typically found in long, needle-like crystals forming fibrous aggregates, often occurring in high-grade metamorphic rocks like schists and gneisses; it is named after the American chemist Benjamin Silliman and is considered a polymorph of kyanite and andalusite, meaning they share the same chemical composition but have different crystal structures; the mineral is often found in regions like India, Sri Lanka, and Myanmar, where it can appear in various colors including white, brown, yellow, green, and blue depending on impurities present