igneous
Biotite-bearing granite or granitic gneiss
Quartz–feldspar–biotite aggregate; likely granite, possibly weathered gneiss
AI-generated identification · may be incorrectCoarse-grained, gray and pink-brown, with glassy quartz, pale feldspar, and dark flaky biotite mica; quartz hardness 7, feldspar 6, biotite 2.5–3, generally nonmetallic with uneven fracture and weak mica cleavage.
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Physical properties
Coarse-grained, gray and pink-brown, with glassy quartz, pale feldspar, and dark flaky biotite mica; quartz hardness 7, feldspar 6, biotite 2.5–3, generally nonmetallic with uneven fracture and weak mica cleavage.
Formation & geological history
Likely formed by slow cooling of silica-rich magma deep underground during Precambrian or younger continental crust formation; the brown surface reflects weathering and iron oxidation. If foliation is present, it may instead be metamorphosed granite (gneiss).
Uses & applications
Commonly used as crushed aggregate, building stone, monuments, and decorative slabs; this small weathered fragment has negligible commercial value.
Geological facts
The contrasting dark mica and light quartz-feldspar minerals are characteristic of felsic continental rocks. Granite commonly weathers to sandy, grainy soil as feldspar alters to clay.
Field identification & locations
Identify it by visible interlocking grains, abundant light minerals, and reflective black mica flakes; it is not likely a gemstone. Common worldwide, especially in continental shields and mountain batholiths.
