metamorphic

Biotite-rich gneiss

Foliated quartz–feldspar–mica gneiss; mainly quartz, plagioclase/alkali feldspar, and biotite

AI-generated identification · may be incorrect

Coarse-grained, strongly foliated rock with dark mica-rich bands and lighter quartz–feldspar patches; minerals have vitreous to pearly luster. Hardness is approximately 6–7 overall, with mica cleavage and irregular to foliated fracture; specific gravity about 2.65–2.80.

Identified September 16, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when preexisting igneous or sedimentary rock underwent high-grade regional metamorphism, causing recrystallization and mineral segregation; individual specimens may range from Precambrian to younger ages.

Uses & applications

Used as crushed stone, aggregate, dimension stone, and occasionally decorative building stone; attractive pieces are collected but are generally low-value.

Geological facts

The alternating light and dark bands are metamorphic compositional layering, not ordinary sedimentary bedding. Biotite flakes commonly produce the black, reflective surfaces visible in the specimen.

Field identification & locations

Identify it by visible foliation, interlocking quartz/feldspar grains, and abundant dark mica; it is common in ancient continental shields and mountain belts worldwide, including Canada, Scandinavia, India, and the Appalachians.