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]