<td">0.002–0.008<td">0.013<td">Pleochroism<td">Blue stones– strong, blue and yellow to colorless. Other colors are weak to very weak.<td">Yellow stones– purplish pink which is stronger in long wave; blue stones– blue to light blue in both long and short wave; green stones– greenish yellow which is stronger in long wave; violet stones– greenish yellow in long wave, light purple in short wave.
|Category||Phosphate mineral group|
|Color||Transparent to translucent, usually green, less often colorless, yellow, blue to violet, pink, brown.|
|Crystal habit||Tabular, prismatic crystals, massive, compact or granular|
|Crystal system||Hexagonal dipyramidal (6/m)|
|Cleavage|| indistinct,  indistinct|
|Fracture||Conchoidal to uneven|
|Mohs Scale hardness||5 (defining mineral)|
|Luster||Vitreous to subresinous|
|Refractive index||1.634–1.638 (+0.012, −0.006)|
|Optical Properties||Double refractive, uniaxial negative|
|Diaphaneity||Transparent to translucent|
Apatite is characteristic of biological systems. It is one of a few minerals produced and used by biological micro-environmental systems. It hardness defines 5 on the Mohs scale. Hydroxyapatite is the main component of tooth enamel and bone mineral.
Much bone material is in a relatively rare form of apatite. In this form most of the OH groups are absent, and there are many carbonate and acid phosphate substitutions.
Fluorapatite (or fluoroapatite) is more resistant to acid attack than is hydroxyapatite. In the mid-20th century it was discovered that communities whose water supply naturally contained fluorine had lower rates of dental caries. Fluoridated water allows exchange in the teeth of fluoride ions for hydroxyl groups in apatite. Similarly, toothpaste often includes a source of fluoride anions (e.g. sodium fluoride, sodium monofluorophosphate).