Calcite

Chemical Formula : CaCO₃
Hardness : 3
Cleavage : Three, perfect, rhombohedral
Luster : Vitreous
Color : Colorless to white when pure, substitutions create yellow, brown, pink, light blue, lavender, light green, gray, black, and reddish brown.
Streak : White
Density : 2.711
Diagnostic Features and Trivia : Calcite is easy to identify by looking for the rhombohedral cleavage, and its strong reaction to cold, dilute HCl. Clear samples of calcite will create double images of an object behind it. These light of these images is polarized at ninety degrees with respect to the other.
Crystal System and Habit : Hexagonal (trigonal); prismatic, tabular on {0001}, rhombohedral, and scalenohedral.
Twinning : Twin plane and composition plane {0001}, or twin plane and composition plane {01-12}.
Polymorphs : Aragonite
Index of Refraction : omega=1.658; epsilon=1.468