Diamond is one such mineral, a fact gem cutters take advantage of often. Color is blue usually but also can be white, gray or green. Color is often not consistent throughout the crystal and can be blochy or in streaks. Luster is vitreous to almost pearly. Transparency crystals are transparent to translucent.
Cleavage is good in one direction parallel to the flat pinacoid face.
Fracture is splintery. Hardness is approximately 4.5 when scratched parallel to the long axis of the crystal and approximately 6.5 when scratched perpendicular to or across the long axis. Streak is white. Associated Minerals are biotite , staurolite , garnets , quartz , andalusite and sillimanite . Best Field Indicators are crystal habit, color, luster and unusual hardness.