metamorphic
Rounded greenstone cobble
Likely greenschist or altered basalt; composition commonly chlorite, epidote, actinolite, albite, and quartz
AI-generated identification · may be incorrectDark green-gray, fine-grained, mottled, and weakly lustrous with a smooth water-worn surface; typical hardness is about 5–6 Mohs, with no visible cleavage or crystals. Exact identification requires a fresh surface, streak, hardness, and mineralogical testing.
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Physical properties
Dark green-gray, fine-grained, mottled, and weakly lustrous with a smooth water-worn surface; typical hardness is about 5–6 Mohs, with no visible cleavage or crystals. Exact identification requires a fresh surface, streak, hardness, and mineralogical testing.
Formation & geological history
Likely formed by low- to medium-grade metamorphism or hydrothermal alteration of basaltic volcanic rock, then rounded by stream or coastal transport. The metamorphic age cannot be determined from the photograph and depends on its source region.
Uses & applications
Usually used as landscaping stone, garden stone, or a decorative cobble; not generally valuable for jewelry or construction aggregate in specimen-sized pieces.
Geological facts
The green color commonly reflects chlorite, epidote, or actinolite produced during alteration. A similarly colored cobble could also be serpentinite or altered basalt, which cannot be separated confidently visually here.
Field identification & locations
Common in glacial deposits, streambeds, beaches, and areas underlain by ancient volcanic or metamorphic rocks. Check for a green streak, hardness relative to glass, magnetic response, and small exposed grains; the pictured specimen is likely worth about $2–$15, mainly as a decorative or local-collected stone.
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