n. (pl. -ies) 1 a hard rock quarried in ancient Egypt, composed of crystals of white or red feldspar in a red matrix. 2 Geol. an igneous rock with large crystals scattered in a matrix of much smaller crystals. porphyritic adj.
[ ME ult. f. med.L porphyreum f. Gk porphurites f. porphura purple ]