metamorphic

Biotite schist (possibly garnet-bearing)

Mica schist; primarily quartz (SiO₂), feldspar, and biotite mica, with possible garnet and amphibole

AI-generated identification · may be incorrect

Dark brown-black, coarse-grained, sparkly micaceous luster with visible foliation and flaky cleavage; typical hardness 2.5–3 for biotite and 6–7 for quartz, specific gravity about 2.7–3.2. The specimen appears weathered and iron-stained.

Identified September 2, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when shale or another clay-rich sediment was subjected to elevated temperature and pressure during regional metamorphism, commonly during mountain building. Its exact geological age cannot be determined from the photograph.

Uses & applications

Used locally as decorative stone, landscaping material, and sometimes dimension stone, though strong foliation limits construction use; attractive pieces are collected as teaching or cabinet specimens.

Geological facts

Schistosity develops as platy minerals rotate and recrystallize under directed pressure, producing the characteristic layered or glittering appearance. Iron oxidation commonly creates brown and rust-colored patches.

Field identification & locations

Look for aligned mica flakes, splitting along foliation, and a glittery surface; a streak, hardness, and acid test would help distinguish it from gneiss or weathered basalt. Common in regional metamorphic belts worldwide, including New England, the Appalachians, Scotland, and the Alps.