igneous
Rounded granite cobble
Granite, chiefly quartz, potassium feldspar, plagioclase, and biotite/hornblende
AI-generated identification · may be incorrectCoarse-grained, speckled pink, white, gray, and black; feldspar gives a pink color, quartz is translucent-gray, and dark minerals are biotite or hornblende. Hardness is about 6–7 Mohs, with a granular interlocking crystal structure, no strong cleavage as a whole, and specific gravity around 2.6–2.7.
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Physical properties
Coarse-grained, speckled pink, white, gray, and black; feldspar gives a pink color, quartz is translucent-gray, and dark minerals are biotite or hornblende. Hardness is about 6–7 Mohs, with a granular interlocking crystal structure, no strong cleavage as a whole, and specific gravity around 2.6–2.7.
Formation & geological history
Intrusive igneous rock crystallized slowly from silica-rich magma deep in Earth’s crust, commonly during Precambrian to younger continental mountain-building events. Its rounded shape indicates transport and abrasion by a river, beach, or glacial environment.
Uses & applications
Used extensively as crushed aggregate, paving stone, building stone, monuments, countertops, and landscaping material; attractive polished pieces may be collected but are usually inexpensive.
Geological facts
The visible pink grains are commonly potassium feldspar, while black grains are mafic minerals. Granite’s interlocking crystals make it durable and resistant to weathering.
Field identification & locations
Identify it by its coarse visible grains and pink-black-white pattern; unlike sandstone, it has no cemented layers or grains. Common worldwide, especially in continental shields and mountain ranges; a hand lens and streak test help distinguish it from similar gneiss.
