igneous
Porphyritic granitic pegmatite
Quartz–feldspar–mica pegmatite, possibly tourmaline-bearing
AI-generated identification · may be incorrectLight gray to cream, coarse crystalline texture with translucent quartz, pale feldspar, and dark brown-black mica or tourmaline inclusions; hardness about 6–7, vitreous to pearly luster, poor cleavage overall but feldspar shows two cleavages. The rounded surface suggests weathering or tumbling.
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Physical properties
Light gray to cream, coarse crystalline texture with translucent quartz, pale feldspar, and dark brown-black mica or tourmaline inclusions; hardness about 6–7, vitreous to pearly luster, poor cleavage overall but feldspar shows two cleavages. The rounded surface suggests weathering or tumbling.
Formation & geological history
Formed from the slow cooling of silica-rich magma, commonly in late-stage granite veins; pegmatites are generally Precambrian to Phanerozoic, depending on locality. Weathering later rounded and exposed the specimen.
Uses & applications
Used decoratively and as a source of quartz, feldspar, mica, and occasionally gemstones such as tourmaline or beryl; attractive pieces are collected rather than used as aggregate.
Geological facts
The very coarse crystals and contrasting dark inclusions are characteristic of pegmatitic rock. A firm identification of the dark minerals requires a fresh surface or testing.
Field identification & locations
Identify by coarse interlocking quartz and feldspar with mica-like or prismatic dark grains; common in granitic terrains worldwide, including Brazil, Madagascar, Maine, and California. Test hardness and cleavage to separate quartz from feldspar.
