Apatite

Found in clusters of nodules, Apatite is a phosphate based mineral that despite its wonderful green colour is something of an orphan stone for most jewellers. Its main drawback being that it has no good planes of cleavage in the crystalline structure, so it doesn't facet well. Though if prepared with care as a cabochon it can show rather nice chatoyancy and (at a push) could pass itself off as lower grade Emerald.

Apatite cabochon