igneous
Biotite-bearing granite or granitic gneiss
Quartz–feldspar granite with biotite mica; possibly weakly foliated gneiss
AI-generated identification · may be incorrectMedium- to coarse-grained, pale quartz and feldspar matrix with black, platy biotite inclusions; vitreous quartz, pearly mica, uneven fracture, and no obvious cleavage in the whole specimen. Typical hardness is about 6–7 Mohs and specific gravity roughly 2.6–2.8.
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Physical properties
Medium- to coarse-grained, pale quartz and feldspar matrix with black, platy biotite inclusions; vitreous quartz, pearly mica, uneven fracture, and no obvious cleavage in the whole specimen. Typical hardness is about 6–7 Mohs and specific gravity roughly 2.6–2.8.
Formation & geological history
Formed from slowly cooled silica-rich magma, likely millions to billions of years ago; if the minerals show planar alignment, later heat and pressure transformed it into granitic gneiss. Common in continental mountain belts and exposed shields.
Uses & applications
Used as crushed aggregate, building stone, countertops, monuments, and landscaping material; attractive polished pieces are collected but are generally not gemstone-grade.
Geological facts
The dark flakes are likely biotite, while the white translucent grains are feldspar and quartz. Rust-colored spots may represent weathering of iron-bearing minerals.
Field identification & locations
Identify it by its interlocking coarse grains, pale quartz/feldspar body, and shiny black mica flakes; a streak or hardness test helps distinguish it from softer dark coatings. Common worldwide, especially in Precambrian shield regions.
