igneous
Granite cobble
Granite, predominantly quartz–feldspar–mica rock
AI-generated identification · may be incorrectCoarse-grained, speckled gray, pink, white, and black; visible interlocking crystals with a vitreous to dull luster. Hardness is about 6–7 Mohs; no prominent cleavage as a whole, though feldspar and mica have cleavage; typical specific gravity is 2.63–2.75.
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Physical properties
Coarse-grained, speckled gray, pink, white, and black; visible interlocking crystals with a vitreous to dull luster. Hardness is about 6–7 Mohs; no prominent cleavage as a whole, though feldspar and mica have cleavage; typical specific gravity is 2.63–2.75.
Formation & geological history
Intrusive igneous rock formed when silica-rich magma slowly crystallized deep underground, commonly during Precambrian to Phanerozoic crustal events. Its rounded shape indicates later transport and abrasion, likely in a river, glacial deposit, or beach gravel.
Uses & applications
Widely used for building stone, countertops, paving, monuments, aggregate, and landscaping. This ordinary cobble has little jewelry or collector value.
Geological facts
The pale crystals are mostly feldspar and quartz, while dark flakes are biotite or amphibole. Granite’s texture records slow cooling because large crystals had time to grow.
Field identification & locations
Identify it by its visibly crystalline, salt-and-pepper texture and hardness; it should resist scratching by a steel knife and not fizz with dilute acid. Common worldwide, especially in exposed continental shields and mountain belts.
