metamorphic
Tumbled banded gneiss or amphibolite
Banded metamorphic rock, likely quartz–feldspar–biotite gneiss with amphibole
AI-generated identification · may be incorrectGray to purplish, fine- to medium-grained, with folded dark and light mineral bands and a dull to slightly silky luster. Typical hardness is about 6–7 Mohs; minerals show no obvious cleavage at this tumbled surface, and specific gravity is roughly 2.7–3.1.
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Physical properties
Gray to purplish, fine- to medium-grained, with folded dark and light mineral bands and a dull to slightly silky luster. Typical hardness is about 6–7 Mohs; minerals show no obvious cleavage at this tumbled surface, and specific gravity is roughly 2.7–3.1.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent intense heat and pressure, producing mineral segregation and folding; commonly Precambrian to Paleozoic in age. The rounded shape appears naturally water-worn or artificially tumbled.
Uses & applications
Used as aggregate, dimension stone, landscaping material, and inexpensive decorative tumbled stone; generally has little gemstone value.
Geological facts
The banding records metamorphic deformation and recrystallization, and the purple-gray tones may reflect biotite, hornblende, quartz, or altered feldspar.
Field identification & locations
Identify it by alternating light and dark mineral bands, granular texture, and lack of vesicles or glassy fracture; common in glacial deposits, stream gravels, and metamorphic terrains worldwide. A streak, hardness test, and fresh broken surface would improve identification.
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