igneous
Biotite-rich granite or granitic gneiss
Granite/granitic gneiss; chiefly quartz (SiO₂), feldspar, and biotite mica
AI-generated identification · may be incorrectCoarse-grained, gray-and-brown, with vitreous quartz, pale feldspar, and shiny black-brown mica; hardness about 6–7 for quartz/feldspar, with mica cleavage and no overall cleavage. Crystals are visibly interlocking, and the specimen appears weakly foliated or weathered.
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Physical properties
Coarse-grained, gray-and-brown, with vitreous quartz, pale feldspar, and shiny black-brown mica; hardness about 6–7 for quartz/feldspar, with mica cleavage and no overall cleavage. Crystals are visibly interlocking, and the specimen appears weakly foliated or weathered.
Formation & geological history
Granite crystallizes slowly from silica-rich magma deep underground; granitic gneiss forms when granite is metamorphosed and banded. Such rocks commonly range from Precambrian to younger ages, depending on locality.
Uses & applications
Used as dimension stone, countertops, monuments, aggregate, and building material; attractive pieces are collected but generally have little gem value.
Geological facts
The contrasting dark mica and light quartz-feldspar minerals are characteristic of continental crust. A streak test is usually white, and dilute acid should produce no fizz.
Field identification & locations
Identify it by coarse interlocking grains, abundant pale feldspar/quartz, and reflective mica flakes; gneiss shows mineral alignment or banding. Common worldwide in continental shields, mountain belts, and exposed basement rocks.
