Sillimanite is sometimes called "Fibrolite" because of the fibrous nature of the material. This is actually pretty rare material, and considering the clarity and color of this particular stone, it's a significant gem. In my experience Sillimanite stones are often very pale in color, almost appearing colorless. Most gems you see on the market are some light shade of blue, green or purple. This stone has a great greenish-yellow color with an "Oval Brilliant" cut and it is only very slightly included.