metamorphic
Biotite-bearing metamorphic rock, likely gneiss or schist
Mica-rich quartz–feldspar gneiss/schist; exact identification requires testing
AI-generated identification · may be incorrectGray to dark brown, coarse-grained, strongly foliated, with reflective black-brown biotite mica and lighter quartz/feldspar. Hardness is approximately 2.5–3 for mica and 6–7 for quartz/feldspar; nonmetallic to pearly luster, with mica cleavage and no obvious uniform crystal form.
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Physical properties
Gray to dark brown, coarse-grained, strongly foliated, with reflective black-brown biotite mica and lighter quartz/feldspar. Hardness is approximately 2.5–3 for mica and 6–7 for quartz/feldspar; nonmetallic to pearly luster, with mica cleavage and no obvious uniform crystal form.
Formation & geological history
Formed when shale or granite was subjected to substantial heat and pressure during mountain-building, typically hundreds of millions to billions of years ago. The apparent cavity may be weathering, a former mineral pocket, or a fracture.
Uses & applications
Used locally as building stone, aggregate, landscaping rock, and decorative slabs; attractive specimens are collected but are generally not jewelry-grade.
Geological facts
Foliation reflects alignment of mica minerals and can cause the rock to split into layers. The silvery flashes are characteristic of mica rather than metal.
Field identification & locations
Look for planar banding or flaky cleavage, abundant mica, and resistance to scratching by a steel knife in quartz-rich areas; a glass-scratch test and close inspection of layering would help distinguish gneiss from schist. Common in Precambrian shields and mountain belts worldwide.
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