Apatite Ore contains information about the Forestry mod which is not included in the current recommended version of Technic Pack and might be outdated..
In Forestry, apatite is the easiest (and only, since 2.x) way to get fertilizer in the early game. Apatite can be found up to slightly above sea level (>60) but is not found in the deepest depths. The vein size ranges from one to eight ore blocks. One Apatite Gem (1-5 gems as of 2.x) is dropped per ore broken.
It is rather easy to spot Apatite ore because it almost has the same shimmering effect of redstone. (Texture changed in Forestry 2.x)