igneous
Rounded granitic gneiss or granite cobble
Biotite granite, possibly weakly foliated granitic gneiss; quartz–feldspar–biotite assemblage
AI-generated identification · may be incorrectHardness about 6–7 Mohs; coarse, granular black, cream, and pink coloration with a speckled texture, vitreous to dull luster, and no obvious cleavage as a whole rock. Quartz and feldspar grains provide resistance to scratching, while biotite forms dark flakes.
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Physical properties
Hardness about 6–7 Mohs; coarse, granular black, cream, and pink coloration with a speckled texture, vitreous to dull luster, and no obvious cleavage as a whole rock. Quartz and feldspar grains provide resistance to scratching, while biotite forms dark flakes.
Formation & geological history
Cooled slowly from silica-rich magma deep underground, likely during Precambrian to Paleozoic continental crust formation; its smooth shape results from prolonged stream, beach, or glacial abrasion.
Uses & applications
Used as aggregate, dimension stone, landscaping rock, and occasionally as a polished decorative specimen; generally has little gem value.
Geological facts
The dark minerals are probably biotite or hornblende, while cream and pink grains are feldspar and translucent grains may be quartz. The rounded surface indicates transport or artificial tumbling rather than its original outcrop form.
Field identification & locations
Identify it by its interlocking coarse grains, abundant light feldspar/quartz, and scattered black mica or amphibole; it should not fizz with dilute acid. Common in glaciated regions and granitic uplands worldwide.
