igneous
Biotite-bearing granite or granitic gneiss
Quartz–feldspar–biotite assemblage; possibly metamorphosed granite (gneiss)
AI-generated identification · may be incorrectCoarse, speckled gray rock with pale feldspar/quartz and reflective black biotite mica; interlocking crystalline texture, vitreous to pearly luster, no obvious cleavage as a whole. Typical hardness is about 6–7 Mohs and specific gravity 2.6–2.8.
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Physical properties
Coarse, speckled gray rock with pale feldspar/quartz and reflective black biotite mica; interlocking crystalline texture, vitreous to pearly luster, no obvious cleavage as a whole. Typical hardness is about 6–7 Mohs and specific gravity 2.6–2.8.
Formation & geological history
Formed from slowly cooling silica-rich magma, or from later high-grade metamorphism of granite; commonly Precambrian to Paleozoic, depending on locality.
Uses & applications
Used as crushed aggregate, dimension stone, building stone, and decorative landscaping material; ordinary specimens have little jewelry value.
Geological facts
The rounded shape appears water-worn or tumbled, and the prominent dark mica flakes distinguish it from many uniform light granites.
Field identification & locations
Common in continental shields, mountain belts, and glacial or river gravels. Identify it by visible quartz, pink/white feldspar, and shiny black mica in a crystalline matrix; a thin section or acid test can confirm composition.
