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From The Collaborative International Dictionary of English v.0.48:
Porphyry \Por"phy*ry\, n.; pl. Porphyries. [F. porphyre, L. porphyrites, fr. Gr. ? like purple, fr. ? purple. See Purple.] (Geol.) 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. [1913 Webster] Porphyry shell (Zool.), a handsome marine gastropod shell (Oliva porphyria), having a dark red or brown polished surface, marked with light spots, like porphyry. [1913 Webster]