Sillimanite, Al2SiO5, Aluminum Silicate, is a polymorph with two other minerals, kyanite and andalusite. A polymorph is a mineral that shares the same chemistry but a different crystal structure with another, or other, minerals. Sillimanites crystal system is orthorhombic. Sillimanite is a rare gemstone that is not frequently seen in faceted form. The crystals are found in shades of brown, white and green. Generally, only the rare, prismatic transparent crystals are cut as gemstones. The refractive index is 1.654-1.683. Sillimanite gemstones have a hardness of 7.5 on the mohs scale, making Sillimanite gemstones very wearable in jewelry. We have the following loose faceted sillimanite gemstones in stock.