Apatite is a material from the Forestry mod. This material is found in large, uncommon veins above sea level, or above Y level 60. It is used to make fertilizer for Forestry farms. When the ore is mined, it will drop a few Apatite gem items.
Originally, Apatite was present in Biomes O' Plenty, but has been renamed to Malachite to avoid confusion with Forestry's Apatite.