metamorphic

Biotite-rich schist (probable mica schist)

Mica schist, dominated by biotite and quartz; possible garnet-bearing variety

AI-generated identification · may be incorrect

Dark brown-black, strongly micaceous and sparkly with rusty weathering; foliated texture and platy cleavage. Typical hardness is 2.5–3 for biotite and about 7 for quartz; vitreous to pearly luster, with variable specific gravity around 2.7–3.2.

Identified September 17, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when clay-rich sedimentary rock underwent medium-grade regional metamorphism, recrystallizing into aligned mica and quartz. It commonly formed during ancient mountain-building events, but the exact age cannot be determined from the image.

Uses & applications

Used locally as decorative stone, landscaping material, aggregate, or architectural facing, though strong foliation limits structural use. Attractive mica-rich specimens are collected, but it is generally not a gemstone.

Geological facts

The glitter comes from reflective biotite mica flakes, while rusty patches likely represent oxidation of iron-bearing minerals. Individual flakes may separate along foliation surfaces.

Field identification & locations

Look for pronounced layered foliation, abundant flexible-looking mica, and a sparkly surface rather than metallic ore. Common in metamorphic mountain belts worldwide, including New England, the Appalachians, Scandinavia, and the Alps.