||Sillimanite is one of the three polymorph minerals, andalusite, sillimanite and kyanite. They have the same chemical formula but different crystal structures. It is rare to find sillimanite crystals clear enough or large enough to facet.
It, like andalusite, is a pleochroic mineral showing two colors in finished stones. It has perfect cleavage in one direction which makes it a bit of challenge for faceters. The colors are typically clear, green, and brown, but occasionally a blue is found.
Because of its fibrous nature some varieties are cut into cabochons with catseyes. It is a rare mineral in jewelry and more often a collectors stone.