igneous
Rounded granitic gneiss or granite cobble
Quartz–feldspar–mica rock; likely granitoid, possibly weakly gneissic
AI-generated identification · may be incorrectCoarse, mottled white, pink, gray, and black grains indicate quartz, feldspar, and biotite/hornblende; vitreous quartz and blocky feldspar are visible. Typical hardness is ~6–7 Mohs, nonmetallic luster, interlocking crystals, and poor cleavage as a whole; rounded edges reflect stream or glacial transport.
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Physical properties
Coarse, mottled white, pink, gray, and black grains indicate quartz, feldspar, and biotite/hornblende; vitreous quartz and blocky feldspar are visible. Typical hardness is ~6–7 Mohs, nonmetallic luster, interlocking crystals, and poor cleavage as a whole; rounded edges reflect stream or glacial transport.
Formation & geological history
Granitoid magma crystallized slowly several kilometers underground during Precambrian–Paleozoic or younger continental crust formation; if gneissic, it was later metamorphosed under high heat and pressure. The specimen was subsequently weathered and transported.
Uses & applications
Commonly used as crushed stone, aggregate, riprap, monuments, and dimension stone; this weathered cobble has little commercial value.
Geological facts
Pink areas are potassium feldspar, pale translucent grains are quartz, and dark flecks are mafic minerals. The surface texture and rounded form are consistent with an ordinary glacial or stream cobble.
Field identification & locations
Look for interlocking coarse grains, abundant quartz, pink/white feldspar, and scattered dark mica; it should not fizz with dilute acid. Common worldwide, especially in glacial tills and river gravels.
