igneous
Polished granite cobble
Granite, typically quartz–feldspar–biotite granite
AI-generated identification · may be incorrectCoarse, interlocking grains with cream/white feldspar and quartz, black biotite or amphibole, and minor reddish minerals; usually Mohs 6–7, vitreous to dull luster, no cleavage as a rock, and specific gravity about 2.6–2.8.
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Physical properties
Coarse, interlocking grains with cream/white feldspar and quartz, black biotite or amphibole, and minor reddish minerals; usually Mohs 6–7, vitreous to dull luster, no cleavage as a rock, and specific gravity about 2.6–2.8.
Formation & geological history
A plutonic igneous rock formed by slow crystallization of silica-rich magma deep in the crust, commonly during Precambrian to Phanerozoic mountain-building events; the rounded shape is from natural abrasion or tumbling.
Uses & applications
Used for countertops, monuments, paving, aggregate, landscaping, and decorative stone; polished cobbles are inexpensive educational or display specimens.
Geological facts
Its visible, interlocking crystals distinguish granite from fine-grained volcanic rocks such as rhyolite; dark veins may represent biotite-rich schlieren or minor mafic inclusions.
Field identification & locations
Identify it by coarse quartz and feldspar crystals, mixed light and dark coloration, and high hardness; common worldwide, especially in continental shields and mountain belts. A knife should not easily scratch it, while quartz may scratch glass.
