Is an Aluminum Silicate with formula Al2 SiO5 .Is also 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. Sillimanite is the rarest of the three trimorphs. The hardness is about 7-7.5 of the Mohs scale. This stone is dark purple-brown.From India.