igneous
Rounded gray granitic cobble
Granite, likely biotite granite or granodiorite; quartz–feldspar–mica aggregate
AI-generated identification · may be incorrectCoarse crystalline, mottled gray-black and white, with a granular texture and weakly visible quartz/feldspar cleavage; individual minerals are typically Mohs 6–7, while the rock has no overall cleavage. Luster ranges from vitreous on quartz to pearly on feldspar; specific gravity is about 2.6–2.8.
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Physical properties
Coarse crystalline, mottled gray-black and white, with a granular texture and weakly visible quartz/feldspar cleavage; individual minerals are typically Mohs 6–7, while the rock has no overall cleavage. Luster ranges from vitreous on quartz to pearly on feldspar; specific gravity is about 2.6–2.8.
Formation & geological history
Formed by slow cooling and crystallization of silica-rich magma deep in the continental crust, commonly during Precambrian to Phanerozoic plutonic events, then exposed and rounded by weathering or transport.
Uses & applications
Used as crushed stone, aggregate, curbing, and building or decorative stone; this small weathered cobble has little jewelry value.
Geological facts
The dark grains are probably biotite or hornblende, while pale grains are feldspar and translucent quartz. The image alone cannot reliably distinguish granite from closely related granodiorite.
Field identification & locations
Common worldwide in continental mountain belts and glacial or river gravels. Identify it by its interlocking visible crystals, quartz sparkle, feldspar whiteness, and black mica; it should not fizz with dilute acid.
