A small glistening particle of talc, selenite, or other crystalline substance present in large numbers in the structure of a rock. Also: a rock or…

A crystal or almond-shaped nodule of some mineral embedded in a trappean rock or the like.

Any of the large or conspicuous crystals in a porphyritic rock, distinct from the groundmass.

Geology. A mineral crystal within a metamorphic rock which has developed its own characteristic crystal faces. Cf. idioblastic, adj. 2.

A relatively large recrystallized grain occurring in a finer groundmass in a metamorphic rock.

A relatively large grain remaining in a finer groundmass in a rock which has undergone dynamic metamorphism.

Geology. A large crystal in a metamorphic rock containing inclusions or small grains of other minerals.