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, but this stone is different from most Sillimanite on the market. This is actually pretty rare material, and one of the few I've seen in this clarity. This gem has a very light lilac-blue color, and the very slight inclusions make it a worthwhile stone. I don't get my hands on many of these stones, and considering that the large majority of the Sillimanite in the world is used for cat's eye stones, clean material is very difficult to find.