igneous
Biotite-rich granite or gneiss
Feldspar–quartz–biotite granitoid; possibly metamorphosed granite (orthogneiss)
AI-generated identification · may be incorrectCoarse, dark gray to brown rock with pale feldspar/quartz and shiny black biotite; crystals are millimeter-scale, with vitreous quartz, feldspar cleavage, and mica cleavage. Typical hardness is about 6–7, specific gravity 2.6–2.8.
Identified More igneous →
Explore Biotite-rich granite or gneiss in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Coarse, dark gray to brown rock with pale feldspar/quartz and shiny black biotite; crystals are millimeter-scale, with vitreous quartz, feldspar cleavage, and mica cleavage. Typical hardness is about 6–7, specific gravity 2.6–2.8.
Formation & geological history
Likely crystallized slowly from silica-rich magma deep underground, commonly Precambrian to Phanerozoic; if gneissic, it was later recrystallized and foliated during regional metamorphism.
Uses & applications
Used as crushed stone, aggregate, building stone, monuments, and countertops; ordinary pieces have little jewelry value.
Geological facts
The dark mineral is probably biotite mica, while the lighter grains are feldspar and quartz. The photograph is insufficient to distinguish massive granite from foliated orthogneiss confidently.
Field identification & locations
Look for interlocking coarse grains, glassy quartz, blocky feldspar, and platy biotite flakes; visible mineral alignment or banding favors gneiss. Common worldwide in continental shields and mountain belts.
