metamorphic
Biotite-bearing metamorphic rock, likely schist
Mica schist; chiefly quartz, feldspar, and biotite mica
AI-generated identification · may be incorrectDark gray-brown, sparkly, flaky and unevenly foliated, with reflective biotite surfaces and pale quartz/feldspar grains. Biotite hardness is about 2.5–3 Mohs, quartz about 7; cleavage is excellent in mica, and overall density is roughly 2.7–2.9 g/cm³.
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Physical properties
Dark gray-brown, sparkly, flaky and unevenly foliated, with reflective biotite surfaces and pale quartz/feldspar grains. Biotite hardness is about 2.5–3 Mohs, quartz about 7; cleavage is excellent in mica, and overall density is roughly 2.7–2.9 g/cm³.
Formation & geological history
Formed when clay-rich sedimentary or igneous rock was buried, heated, and compressed during mountain-building, recrystallizing into aligned mica and quartz. Its exact geological age cannot be determined from the photograph; schists range from Precambrian to modern mountain belts.
Uses & applications
Used locally as decorative stone, landscaping material, and occasionally dimension stone, but strong foliation limits construction use. Attractive mica-rich pieces are mainly collected as geological specimens.
Geological facts
The glitter comes from mica cleavage surfaces rather than metallic crystals. A hand lens can reveal flexible, paper-thin biotite flakes and possible garnet or pyrite inclusions.
Field identification & locations
Common in metamorphic belts worldwide, including the Appalachians, New England, the Alps, and western North America. Identify it by pronounced foliation, sparkly black mica, and light quartz-rich layers; a photo cannot confirm the exact variety.
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