Biotite-rich schist

Dark gray-brown, strongly foliated, sparkly micaceous surface with uneven cleavage; quartz grains may appear glassy. Hardness is about 2.5–3 for biotite and 7 for quartz; specific gravity commonly 2.7–3.0.

Formed when clay-rich sedimentary rock was subjected to regional metamorphism, recrystallizing into aligned mica and quartz. Usually Paleozoic to Precambrian in exposed mountain belts, though age depends on locality.

Used locally as decorative stone, landscaping rock, aggregate, and occasionally building veneer; generally low jewelry value because it splits and weathers readily.

Its glitter comes from reflective mica cleavage faces, not metallic ore. Foliation records directed pressure during mountain building.

Identify by thin, parallel layers that flake, abundant shiny black/brown mica, and a lack of uniform metallic luster; a streak or hardness test can distinguish pyrite-rich rock. Common in metamorphic belts worldwide, including New England, the Appalachians, Scotland, India, and the Alps.