igneous
Black-and-white granitoid pebble
Likely granite or granodiorite; quartz + feldspar + dark biotite/amphibole
AI-generated identification · may be incorrectCoarse, irregular interlocking crystals with black mafic minerals and white/gray feldspar or quartz; vitreous to dull luster, no obvious cleavage as a whole, Mohs hardness about 6–7, and specific gravity roughly 2.6–2.8.
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Physical properties
Coarse, irregular interlocking crystals with black mafic minerals and white/gray feldspar or quartz; vitreous to dull luster, no obvious cleavage as a whole, Mohs hardness about 6–7, and specific gravity roughly 2.6–2.8.
Formation & geological history
An intrusive igneous rock that crystallized slowly from silica-rich magma deep in the crust, commonly during Precambrian to Phanerozoic continental mountain-building; later weathering and transport rounded it into a pebble.
Uses & applications
Used as crushed aggregate, building and monument stone, landscaping rock, and occasionally polished for decorative objects; this nonspecific specimen has little jewelry value.
Geological facts
The strong black-and-white contrast reflects abundant biotite or amphibole against light feldspar and quartz. Exact identification requires a fresh surface, hand lens, and tests for quartz, feldspar, and magnetic minerals.
Field identification & locations
Common in continental crust and glacial or river gravels worldwide, especially near granitic terrains. Identify it by coarse visible crystals, hard quartz-rich grains, and lack of layering or foliation; it may resemble gneiss if mineral bands are present.
