porphyry

n. (pl. -ies) hard rock composed of crystals of white or red feldspar in a red matrix.  porphyritic adj. [greek: related to *purple]

n. 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.