metamorphic

Gneissic granite cobble

Granitic gneiss; quartz–feldspar–biotite assemblage

AI-generated identification · may be incorrect

Coarse-grained, speckled black, gray, cream, and tan rock with weakly banded or swirled texture; quartz and feldspar provide a vitreous to dull luster, while biotite is darker and flaky. Hardness is approximately 6–7 Mohs; cleavage is absent overall, though feldspar and biotite have mineral cleavage.

Identified September 12, 2026More metamorphic →

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Physical properties

Formation & geological history

Likely metamorphosed granite: intense heat and pressure recrystallized and partially aligned its minerals during Precambrian to Paleozoic mountain-building. Its rounded form indicates later transport as a stream or glacial cobble.

Uses & applications

Used as durable crushed stone, aggregate, riprap, landscaping rock, and occasionally dimension stone; generally little jewelry value unless polished for decorative use.

Geological facts

Gneiss commonly preserves the original mineral chemistry of granite while developing compositional banding. The visible light minerals are mainly quartz and feldspar, with dark biotite or hornblende.

Field identification & locations

Identify it by coarse interlocking grains, abundant light quartz-feldspar zones, and dark mica/amphibole bands or patches; granite lacks the stronger mineral segregation typical of gneiss. Common in ancient continental shields and glaciated terrains worldwide.