igneous
Biotite-rich granite or granitic gneiss
Granite, chiefly quartz + feldspar + biotite mica; possibly weakly gneissic
AI-generated identification · may be incorrectMedium- to coarse-grained, dark gray and speckled with reflective black-brown biotite; likely Mohs 6–7 overall, vitreous quartz/feldspar luster, interlocking crystalline texture, and poor/no cleavage as a whole. The rounded shape appears tumbled or naturally water-worn.
Identified More igneous →
Explore Biotite-rich granite or granitic gneiss in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Medium- to coarse-grained, dark gray and speckled with reflective black-brown biotite; likely Mohs 6–7 overall, vitreous quartz/feldspar luster, interlocking crystalline texture, and poor/no cleavage as a whole. The rounded shape appears tumbled or naturally water-worn.
Formation & geological history
Formed by slow cooling of silica-rich magma deep in the continental crust, commonly during Precambrian to Phanerozoic mountain-building events; if gneissic, it was later subjected to high heat and pressure.
Uses & applications
Common granite is used for crushed stone, aggregate, monuments, countertops, and building stone; attractive polished pieces may be sold as decorative or educational specimens.
Geological facts
The bright flakes are probably biotite mica, while pale grains are feldspar or quartz. Exact identification is limited by the small, rounded specimen and lighting.
Field identification & locations
Look for visible interlocking quartz and feldspar grains, black platy mica, hardness greater than glass, and no reaction to dilute acid. Common worldwide in continental shields and mountain belts.
