igneous
Biotite-bearing granite or granitic gneiss
Quartz–feldspar–biotite rock; probable granite (locally gneissic)
AI-generated identification · may be incorrectHardness about 6–7; coarse-grained gray rock with white feldspar/quartz and dark, flaky biotite or amphibole, showing a glassy-to-pearly luster. Interlocking crystals are typical; cleavage is mainly in the mica grains, and specific gravity is approximately 2.6–2.8.
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Physical properties
Hardness about 6–7; coarse-grained gray rock with white feldspar/quartz and dark, flaky biotite or amphibole, showing a glassy-to-pearly luster. Interlocking crystals are typical; cleavage is mainly in the mica grains, and specific gravity is approximately 2.6–2.8.
Formation & geological history
Likely crystallized slowly from silica-rich magma deep underground, or represents metamorphosed granite if the minerals are foliated. Such rocks commonly formed in Precambrian to Mesozoic continental crust, depending on locality.
Uses & applications
Widely used as crushed stone, aggregate, road material, building stone, and occasionally countertops or monuments; this weathered specimen has little gem value.
Geological facts
The dark patches are probably biotite mica and/or hornblende, while white areas are feldspar and quartz. A glassy quartz grain, visible mica flakes, and a scratch resistance near that of steel are useful clues.
Field identification & locations
Common in continental mountain belts and glacial deposits worldwide, including North America, Europe, India, and Australia. Examine a fresh broken surface and test for visible quartz, feldspar cleavage, and mica flakes; exact identification requires petrographic or chemical testing.
