metamorphic
Foliated metamorphic cobble, likely gneiss
Biotite–quartz–feldspar gneiss (tentative)
AI-generated identification · may be incorrectDark gray, rounded beach cobble with pale, irregular mineral banding and a subdued sparkle from mica; likely Mohs 6–7, with granular texture, weak foliation, and no obvious cleavage at this scale.
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Physical properties
Dark gray, rounded beach cobble with pale, irregular mineral banding and a subdued sparkle from mica; likely Mohs 6–7, with granular texture, weak foliation, and no obvious cleavage at this scale.
Formation & geological history
Formed when quartzofeldspathic sedimentary or igneous rock underwent high-temperature regional metamorphism, commonly during Precambrian–Paleozoic mountain building; later erosion produced the rounded cobble.
Uses & applications
Gneiss is used as crushed stone, aggregate, dimension stone, and occasionally countertops or landscaping rock. This beach-worn specimen has mainly modest collecting value.
Geological facts
The light bands are probably quartz and feldspar, while darker areas may contain biotite, amphibole, or other mafic minerals. Exact identification requires a fresh surface, hand lens, and ideally thin-section or chemical testing.
Field identification & locations
Look for alternating light and dark mineral bands, interlocking grains, and hardness sufficient to scratch glass; unlike granite, gneiss commonly shows directional mineral alignment. Common in glaciated terrains, mountain belts, and beaches downstream from exposed bedrock.
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