metamorphic
Biotite-bearing schist (likely mica schist)
Mica schist, commonly quartz + biotite ± muscovite; composition varies
AI-generated identification · may be incorrectDark gray to brown, strongly foliated, sparkly micaceous luster with flaky layers and irregular cleavage; typical hardness 2–3 for mica and about 7 for quartz, specific gravity roughly 2.7–2.9. The visible foliation and glittering platy grains distinguish it from unfoliated basalt.
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Physical properties
Dark gray to brown, strongly foliated, sparkly micaceous luster with flaky layers and irregular cleavage; typical hardness 2–3 for mica and about 7 for quartz, specific gravity roughly 2.7–2.9. The visible foliation and glittering platy grains distinguish it from unfoliated basalt.
Formation & geological history
Formed when shale or other clay-rich sedimentary rock underwent medium-grade regional metamorphism, recrystallizing into mica and quartz; commonly Paleozoic to Precambrian, though age depends on locality.
Uses & applications
Used locally as crushed stone, fill, wall stone, and decorative landscaping rock; attractive slabs may be used for architectural veneer, but it is generally not a jewelry gemstone.
Geological facts
Mica flakes reflect light and can separate along foliation; weathering commonly produces rusty brown staining from iron-bearing minerals.
Field identification & locations
Identify it by thin, aligned, glittering mica layers and a tendency to split into plates; common in metamorphic belts worldwide, including the Appalachians, New England, Scotland, India, and Brazil. A photograph cannot confirm the exact variety without a fresh break or testing.
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