igneous
Granite (likely biotite granite)
Coarse-grained felsic plutonic rock: quartz + feldspar + biotite; approximate composition SiO₂-rich
AI-generated identification · may be incorrectHardness about 6–7 Mohs; pale gray-white with black biotite flakes, granular texture, vitreous quartz and feldspar luster, no foliation, and poor cleavage as a rock. The rounded surface is naturally weathered or water-worn.
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Physical properties
Hardness about 6–7 Mohs; pale gray-white with black biotite flakes, granular texture, vitreous quartz and feldspar luster, no foliation, and poor cleavage as a rock. The rounded surface is naturally weathered or water-worn.
Formation & geological history
Formed when silica-rich magma slowly crystallized deep underground, producing interlocking crystals; commonly Precambrian to Phanerozoic in age. Subsequent uplift and erosion exposed and rounded it.
Uses & applications
Widely used as dimension stone, countertops, monuments, paving, aggregate, and landscaping rock; attractive specimens are collected but usually have modest value.
Geological facts
The black grains are probably biotite mica, though hornblende or other dark minerals may occur. Its visible interlocking crystals and lack of layering distinguish it from gneiss and many sedimentary rocks.
Field identification & locations
Identify it by its speckled, interlocking quartz-feldspar-mica texture and inability to scratch easily with a steel knife. Common worldwide, especially in continental mountain belts; fresh surfaces reveal the grain better.
