The composition of sillimanite is Al[AlSiO5], and it crystallizes in an orthorhombic crystal system. The crystals are acicular, usually in the form of radiating fibrous aggregates. The colour is grey, brown or greyish green. It has a vitreous or silky lustre and uneven fractures. Its specific gravity is 3.23–3.25, and its Mohs hardness is 7. Sillimanite is a standard mineral of metamorphic rocks and is common in intrusive rocks, contact metamorphic zones with aluminium-rich rocks, and crystalline schist and gneiss.