Sillimanite is a high temperature and high pressure aluminum silicate found in high-grade regional metamorphic rocks. Transparent crystals are called "fibrolite" by gemologists. This is a gemmy, fine, light purple elongate crystal with a bit of rounding. The edge rounding likely represents resorption into the matrix more than stream wear. This example from Mogok, Burma is sharper than most. Ex. Irv Brown Collection. 5.06 carats.