metamorphic
Banded gneiss pebble
Gneiss, composed chiefly of quartz, feldspar, and mica/amphibole
AI-generated identification · may be incorrectHardness about 6–7 Mohs; coarse-grained, with irregular black, white, gray, and rusty-brown bands. Nonmetallic vitreous to dull luster; foliation is visible, though the rounded surface obscures cleavage.
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Physical properties
Hardness about 6–7 Mohs; coarse-grained, with irregular black, white, gray, and rusty-brown bands. Nonmetallic vitreous to dull luster; foliation is visible, though the rounded surface obscures cleavage.
Formation & geological history
Formed when granite or other felsic rock underwent intense heat and pressure, producing compositional banding and recrystallization. It may be Precambrian or younger; the specimen’s exact age cannot be determined from the image.
Uses & applications
Used as dimension stone, paving, landscaping rock, and aggregate; attractive polished pieces are collected as decorative specimens.
Geological facts
The dark bands commonly contain biotite, hornblende, or other ferromagnesian minerals, while light bands are quartz- and feldspar-rich. Rusty colors indicate oxidation of iron-bearing minerals.
Field identification & locations
Identify it by alternating light and dark mineral bands, coarse crystals, and strong density; unlike granite, its minerals show preferred alignment. Common in continental shields, mountain belts, and glacial or stream gravels.
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