What does "porphyry" mean?

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