Apatite (specifically Fluorapatite)
Chemical Formula : Ca5(PO4)3(F,Cl,OH)
Hardness : 4
Cleavage : One, poor
Luster : Vitreous to greasy
Color : Greenish yellow, bluish green, grayish green, pale pink, or
violet
Streak : White
Density : 3.17-3.23
Diagnostic Features and Trivia : Apatite is the definition of a hardness
of 4 on Moh's scale. It also has the same composition as teeth. The name
however has nothing to do with eating. The greek root for the word means
"deceit" and comes from the similar appearance of apatite to
olivine and fluorite.
Crystal System and Habit : Hexagonal; commonly thick tabular on {0001}
with pipyramid forms.
Twinning : Relatively common on {21-31} and {12-31}
Polymorphs : None
Index of Refraction : omega=1.632; epsilon=1.628