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Apatite (calcium phosphate)
Color: blue, green, yellow, pink, violet, colorless.
Moh’s hardness: 5
Refractive index: 1.632-1.646
Critical angle: 37.5°
Specific gravity: 3.17-3.23
Cleavage: Poor, imperfect.
Fracture: Conchoidal, brittle.
Dispersion: Medium (.013-.016)
Heat sensitivity: High
Pleochroism: Dichroic – Blue: very strong (blue to weak-blue or colorless); Green: weak (green to yellow-green); Yellow: weak (gold-yellow to green-yellow).
Birefringence (double refraction): Weak (.002-.004)
Crystal structure: Hexagonal
Treatments: Apatite may be heat treated to bring out a better blue color and to remove silk.
Cutting: Apatite is brittle, so roughing in should be done with care.
Polishing: Can be difficult to polish. Use wax or tin/lead lap with alumina or cerium oxide; or chrome or tin oxide Ultralaps. Vinegar may ease polishing difficulties.
Comments: Because of apatite’s heat sensitivity, dopping with glue is safer.