Biotite-rich schist (mica schist)

Typically gray-brown, strongly sparkly, and foliated, with platy mica flakes and a schistose texture; hardness about 2.5–3 for biotite and 6–7 for quartz, with vitreous to pearly luster and mica cleavage.

Forms when shale or other clay-rich sediment undergoes medium-grade regional metamorphism, commonly during mountain building; ages vary widely from Paleozoic to Precambrian.

Used locally as decorative stone, landscaping material, and occasionally dimension stone, but its pronounced foliation limits structural use. Attractive mica-rich pieces are collected as specimens.

The glitter comes from aligned biotite or muscovite flakes reflecting light; foliation records directed pressure and deformation during metamorphism.

Identify it by abundant reflective mica, layered/fissile surfaces, and visible quartz or feldspar grains; unlike glittery granite, the minerals are preferentially aligned and the rock splits along foliation. Common in metamorphic mountain belts worldwide, including New England, the Appalachians, the Alps, and Scandinavia.