Apatite

As the reference mineral for hardness 5 on the Mohs scale, apatite occupies the middle of that comparative sequence. The name denotes a group of calcium phosphate minerals with the general formula Ca5(PO4)3(F,Cl,OH) that crystallize in the hexagonal system. Specimens are transparent and vitreous and range widely in color through green, blue, yellow, violet, and colorless, with a white streak and poor basal cleavage. Beyond its mineralogy, apatite is biologically fundamental: it serves as the principal reservoir of phosphorus for living systems and forms both tooth enamel and the inorganic fraction of bone. Prismatic and tabular crystals are the usual habit, and the specific gravity falls between 3.16 and 3.22.

Apatite refers to a group of phosphate minerals and is the reference mineral for hardness 5 on the Mohs scale. It is the main source of phosphorus for biological systems and is the mineral that makes up tooth enamel and bone mineral.