Mica schist

A foliated metamorphic rock with glittering platy mica, dark brown-black biotite, and lighter silvery muscovite; quartz and feldspar may form granular bands. Mica has Mohs hardness 2–3 and perfect basal cleavage, while quartz is about 7; the specimen shows a flaky, reflective schistosity rather than distinct crystal faces.

Formed when shale or a clay-rich sediment was subjected to regional metamorphism, typically during mountain building. Its exact age and locality cannot be determined from the photograph; schists range from Precambrian to Phanerozoic ages.

Used locally as decorative stone, landscaping material, and occasionally building facing; generally unsuitable for high-wear construction because foliation splits readily. Attractive mica-rich pieces are collected as educational specimens.

The strong sheen is produced by aligned mica flakes reflecting light. Weathering commonly makes schist surfaces rusty brown through oxidation of iron-bearing biotite.

Identify it by pronounced foliation, easy splitting along mica-rich layers, and sparkly platy cleavage; distinguish it from slate by its coarser visible crystals and from phyllite by stronger mica sparkle. Common in regional metamorphic belts worldwide, including the Appalachians, Alps, Himalaya, and New England.