Porphyry

[Sarah]

A term used somewhat loosely to designate a rock consisting ofa fine-grained base (usually feldspathic) through which crystals, asof feldspar or quartz, are disseminated. There are red, purple, andgreen varieties, which are highly esteemed as marbles. Porphyry shell(Zoöl.), a handsome marine gastropod shell (Oliva porphyria), havinga dark red or brown polished surface, marked with light spots, likeporphyry.