Dolomite
Chemical Formula : CaMg(CO3)2
Hardness : 3.5-4
Cleavage : Three, perfect, rhombohedral
Luster : Vitreous to pearly
Color : Colorless, white, gray, greenish, or pink.
Streak : White
Density : 2.85
Diagnostic Features and Trivia : Dolomite can be identified by its rhombohedral cleavage, hardness, and reaction to warm HCl. It will react weakly with cool HCl if powdered.
Crystal System and Habit : Hexagonal (trigonal); common crystals are rhombohedral, prismatic, or tabular. Often surfaces are curved creating saddle shapes.
Twinning : Common on {0001}, {10-10}, and {11-20}
Polymorphs : None
Index of Refraction : omega=1.679; epsilon=1.5