Fluorite

Chemical Formula : CaF₂
Hardness : 4
Cleavage : Four, perfect
Luster : Vitreous
Color : Colorless and transparent if pure, otherwise any color is possible with zoning related to varying level of impurities common.
Streak : White
Density : 3.18
Diagnostic Features and Trivia : Fluorite is the definition of hardness level 4 on Moh's scale. It is also easy to determine its identity because of its octahedral structure from cleavage.
Crystal System and Habit : Isometric; commonly forming cubes.
Twinning : Twin axis [111] forming interpenetrating cubes.
Polymorphs : None
Index of Refraction : 1.433-1.435