igneous
Biotite-bearing granite or granitic gneiss
Quartz–feldspar–biotite rock; likely granitoid, possibly foliated gneiss
AI-generated identification · may be incorrectCoarse, speckled black-andcream specimen with vitreous gray quartz, pale feldspar, and black biotite mica. Typical hardness is about 6–7 Mohs; quartz is glassy, feldspar shows cleavage, and biotite has perfect sheet cleavage. Exact identification is limited by the small, rounded, possibly polished sample.
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Physical properties
Coarse, speckled black-andcream specimen with vitreous gray quartz, pale feldspar, and black biotite mica. Typical hardness is about 6–7 Mohs; quartz is glassy, feldspar shows cleavage, and biotite has perfect sheet cleavage. Exact identification is limited by the small, rounded, possibly polished sample.
Formation & geological history
Granite crystallizes slowly from silica-rich magma deep in the crust, commonly during Precambrian to Mesozoic intrusive events; granitic gneiss forms when similar material is metamorphosed and banded. Common sources include continental mountain belts and ancient cratons.
Uses & applications
Used as building stone, aggregate, countertops, monuments, and decorative specimens; attractive pieces may be collected but are usually low-value.
Geological facts
Black flakes are likely biotite, while translucent gray grains are quartz and cream grains are feldspar. A true gneiss should show mineral banding or alignment rather than a random granular texture.
Field identification & locations
Identify it by its interlocking coarse grains, visible quartz, light feldspar, and platy black mica; test with a steel blade and inspect for foliation. Common worldwide, especially in granite quarries and metamorphic terrains.
