metamorphic
Dark gray slate or phyllite cobble
Slate–phyllite, composed mainly of fine-grained mica, quartz, and chlorite
AI-generated identification · may be incorrectTypically dark gray to blue-gray, fine-grained, with a silky to dull luster and strong planar cleavage; Mohs hardness is about 3–5, and specific gravity roughly 2.7–2.8. The visible pale linear feature may be a mineral vein or cleavage-related fracture.
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Physical properties
Typically dark gray to blue-gray, fine-grained, with a silky to dull luster and strong planar cleavage; Mohs hardness is about 3–5, and specific gravity roughly 2.7–2.8. The visible pale linear feature may be a mineral vein or cleavage-related fracture.
Formation & geological history
Formed when shale or mudstone underwent low-grade regional metamorphism during mountain building, commonly hundreds of millions of years ago. The rounded shape indicates later transport and abrasion in a river, beach, or glacial deposit.
Uses & applications
Used for roofing, floor and wall tiles, paving, landscaping, and aggregate; attractive pieces may be used as garden stones or decorative specimens. This cobble has little commercial value unless its texture or provenance is unusual.
Geological facts
The specimen appears more massive and less sharply fissile than typical roofing slate, so phyllite or metamorphosed mudstone is also plausible. Its exact identity cannot be confirmed from an image alone.
Field identification & locations
Look for fine foliation, planar splitting, a dull or silky surface, and absence of visible large crystals; a streak test, thin-section study, or fresh broken surface would help distinguish slate from phyllite. Common in metamorphic belts and sediment-rich stream or beach gravels.
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