igneous
Weathered granite or granitic gneiss
Granite; chiefly quartz, feldspar, and biotite/hornblende
AI-generated identification · may be incorrectCoarse, mottled tan-gray specimen with visible light feldspar/quartz and dark mafic minerals; likely Mohs 6–7, generally nonmetallic, with no obvious cleavage as a whole rock. The elongated or streaked texture may indicate weak gneissic banding, but the image is insufficient for a definitive distinction.
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Physical properties
Coarse, mottled tan-gray specimen with visible light feldspar/quartz and dark mafic minerals; likely Mohs 6–7, generally nonmetallic, with no obvious cleavage as a whole rock. The elongated or streaked texture may indicate weak gneissic banding, but the image is insufficient for a definitive distinction.
Formation & geological history
Granite crystallizes slowly from silica-rich magma deep in the crust; granitic gneiss forms when granite is metamorphosed under high temperature and pressure. Such rocks commonly range from Precambrian to Mesozoic in age, depending on locality.
Uses & applications
Used as crushed aggregate, building stone, paving, monuments, and countertops; attractive polished pieces may be collected but are usually inexpensive.
Geological facts
Quartz is typically glassy and feldspar commonly shows blocky cleavage surfaces; dark biotite or hornblende produces the speckled appearance. Weathering often turns feldspar surfaces yellow-brown.
Field identification & locations
Common in continental mountain belts and exposed shields worldwide, including North America, Scandinavia, India, and Australia. A fresh broken surface, hand lens, and acid test (no fizz) help distinguish it from limestone; visible foliation would support gneiss.
