Apatite Ore is an ore added by the Forestry mod, it is based on a phosphate mineral and is used primarily as a manufacturer of Fertilizer (Forestry). It can also be used as a gemstone.
As of Forestry 2.0 Apatite Ore only occurs up above level 60 (level 64 is sea level). Up to 48 blocks can be found in one vein. One apatite gem is dropped per ore block mined.