Sillimanite is actually trimorphous with Kyanite and Andalusite which means that they have the same chemistry, but different crystal habits create different minerals, just like Brookite, Anatase and Rutile. Sillimanite is sometimes called "Fibrolite" because of the fibrous nature of the material. This is actually pretty rare material, and one of the few I've seen in this color. This stone has a very distinct "eye" and a really unusual "smoky" color. The stone has decent clarity as well. I photographed this stone with a strong penlight shining right on the stone to show the "eye" better, so it might not be as distinct depending on what lighting in which you're viewing the stone.