Sillimanite is an aluminum silicate that usually occurs in rocks that are rich in clay and have undergone regional metamorphism at high temperatures, such as gneisses. The mineral varies in color, but often appears glassy white, green, or brown. There also exists, however, a light blue form of sillimanite that can be found in Sri Lanka, which is sometimes utilized as a gemstone. Typically long and slender in shape, crystals of sillimanite frequently occur in fibrous masses, but are generally too brittle to be considered asbestos. Many minerals are associated with sillimanite, including quartz, biotite, feldspars, and garnets.