Is a Calcium (Fluoro,Chloro,Hydroxyl) Phosphate with formula Ca5(PO4)3(OH,F,Cl) and hardness of 5
Mostly is not used in Jewellery due to his low hardness (5 from the Mohs scale), sensitive to heat and acid (contain often calcium fluoride, like ours teeth).
We can find apatite in large variety of colours. The blue is the most appreciated and collected. Sizes over 5 carats are very rare.This is the blue-green color from Madagascar. Eye clean.