Apatite, at 5 on the Mohs scale, is the defining mineral for that hardness. Although apatite is abundant in the earth, it is very rare to find any that is fine enough quality for gems. The gem quality apatite is typically blue to blue-green to green, and the very best is translucent to transparent.
Just a point of interest- apatite is a main component of teeth, and is a mineral used in fertilizer.
