metamorphic

Dark gray metamorphic cobble, likely slate or phyllite

Foliated metapelite; possibly slate/phyllite composed mainly of quartz, mica, and chlorite

AI-generated identification · may be incorrect

Fine-grained, dark gray, dull to weakly silky luster, with subtle foliation and irregular cleavage; typical hardness is about 3–6 Mohs, depending on mica and quartz content. The image does not show enough detail for a definitive species identification.

Identified September 4, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when clay-rich sedimentary rock was regionally metamorphosed under low-grade conditions, commonly during mountain building; age depends on its source terrain. It appears to be a naturally rounded cobble placed on or beside a concrete surface.

Uses & applications

Slate and phyllite are used for roofing, paving, wall cladding, aggregate, and decorative stone; this small specimen has mainly educational or collecting value.

Geological facts

Slate splits into thin, planar sheets, while phyllite has a more pronounced silky sheen and wavy foliation. Dark coloration commonly reflects carbonaceous material, chlorite, or fine-grained iron-bearing minerals.

Field identification & locations

Check for planar splitting, aligned mica flakes, and a streak or foliation that persists across the specimen; a fresh broken surface is more diagnostic. Common sources include metamorphic belts in Appalachia, New England, Wales, Spain, and the Alps.