metamorphic
Rounded granitic gneiss cobble
Foliated quartz–feldspar–mica gneiss; likely granitoid-derived
AI-generated identification · may be incorrectCoarse, mottled tan-gray rock with dark biotite/amphibole-rich bands and rounded stream-worn surfaces; hardness about 6–7, vitreous to dull luster, interlocking crystalline texture, and weak foliation. Specific gravity is approximately 2.6–2.8.
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Physical properties
Coarse, mottled tan-gray rock with dark biotite/amphibole-rich bands and rounded stream-worn surfaces; hardness about 6–7, vitreous to dull luster, interlocking crystalline texture, and weak foliation. Specific gravity is approximately 2.6–2.8.
Formation & geological history
Formed when granitic protolith was subjected to intense heat and pressure, producing gneissic banding; its exact age cannot be determined visually, but such rocks commonly formed during Precambrian to Paleozoic mountain-building events. The rounded shape indicates transport by water or glacial action.
Uses & applications
Used as decorative landscaping stone, riprap, retaining-wall material, and occasionally cut as building stone; attractive pieces may be collected but are generally low-value.
Geological facts
The dark patches are likely biotite, hornblende, or weathered mafic minerals, while lighter areas contain quartz and feldspar. Surface iron staining produces the rusty brown tones.
Field identification & locations
Identify it by coarse visible grains, alternating light and dark mineral concentrations, strong toughness, and lack of sedimentary layering or vesicles; common in glaciated and mountainous regions worldwide.
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