Sillimanite is a relatively common aluminosilicate metamorphic rock forming mineral and very rarely forms in gem crystals like this gemmy and lustrous thumbnail! This slender prismatic, medium blue-gray crystal is gem from top to bottom displaying a good adamantine luster. It is certainly terminated on one end and not on the other. The faces and edges of this Sillimanite crystal are well formed with distinct striations present on the prism faces parallel to the length of the crystal. This little Sillimanite gem displays noticeable pleochroism on a white iPhone screen that varies from blue to pinkish blue in perpendicular orientations. 4.75 carats