Sillimanite is a relatively common metamorphic rock forming mineral, however, it very rarely forms in gem crystals like this amazing thumbnail! This elongated and slender prismatic, medium blue-gray crystal is gem throughout its length and has a touch of a satin luster except where there is a cleavage trace on the back half of the crystal that is distinctly adamantine. It is terminated on one end and partially so on the other end and the edges of the crystal are slightly rounded. This rounding is not uncommon for these and other species like Spinel that are often frozen in the metamorphic rock host matrix and if weathered out can become more rounded by stream transport. This little Sillimanite gem displays noticeable pleochroism on a white iPhone screen that varies from blue to pinkish blue in perpendicular orientations. 6.55 carats