Sillimanite is one of three Alumino-Silicate Polymorphs, the other two being Andalusite and Kyanite. A common variety of Sillimanite is known as Fibrolite, so named because the Mineral appears like a bunch of fibers twisted together when viewed under thin section or even by the naked eye. Both the fibrous and traditional forms of Sillimanite are common in Metamorphosed Sedimentary Rocks.