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, lavender, 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. 7.22 carats.