porphyry

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Rock containing relatively large conspicuous crystals, especially feldspar, in a fine-grained igneous matrix.

Century Dictionary and Cyclopedia

1. n. The English form of Latin word porphyrites, used by the Romans to designate a certain rock having a dark-crimson ground through which are scattered small crystals of feldspar. In Pliny's time this rock, which was quarried in Egypt, was used extensively for architectural and ornamental purposes, and especially for the base or lower part of busts of which the upper part was made of bronze or marble. Later on, a similar stone appears to have been procured from nearer localities, as from the island of Sardinia. To the Italians it became known as porfido rosso antico. Other rocks having a similar structure, commonly called porphyritic, were used in Italy, and designated, in accordance with the predominating color, as porfido nero, porfido verde, etc. In modern times the term porphyry has come to be used as the name of any rock consisting of a very fine grained or microcrystalline ground-mass through which are disseminated distinctly recognizable crystals of some mineral; but the popular use of the word is frequently extended so as to include rocks which are dark-colored, fine-grained, and very hard, and which do not appear to belong either to the marbles or granites, and this is done even when the porphyritic structure is not at all or only very indistinctly marked. The varieties of porphyry are numerous, and their nomenclature by no means definitely established. The most generally accepted are the following: quartz-porphyry, of which the ground-mass consists of an intimate or cryptocrystalline admixture of orthoclase and quartz, in which distinct crystals or large grains of quartz are developed; feldspar, felsitic or felstone porphyry, having a similar base with porphyritically inclosed crystals of feldspar, which is commonly orthoclase; but similar crystals of this mineral are not infrequently found occurring with the quartz in quartz porphyry, so that no very distinct line can be drawn separating the two varieties mentioned. These porphyries are of most frequent occurrence in the Paleozoic rocks, but they are also found in abundance in other Pre-tertiary formations, presenting the characters of a truly eruptive material. See porphyrite, and cut under porphyritic.
2. n. A slab of porphyry, used in alchemy.
3. n. In zoology, a porphyry-moth.
4. n. In ceramics, a hard colored body made by Josiah Wedgwood, in imitation of porphyry.

Wiktionary

1. n. geology a hard igneous rock consisting of large crystals in a fine-grained matrix

GNU Webster's 1913

1. n. (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.

WordNet 3.0

1. n. any igneous rock with crystals embedded in a finer groundmass of minerals