metamorphic
Biotite schist (likely graphite-bearing)
Mica schist; chiefly quartz + biotite mica, with possible graphite and feldspar
AI-generated identification · may be incorrectDark gray to black, strongly foliated and sparkly, with platy mica grains and pale quartz-rich streaks. Typical hardness is about 2.5–3 for biotite and 6–7 for quartz; mica has one perfect cleavage, while the rock has pronounced schistosity and moderate density (~2.7–3.0 g/cm³).
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Physical properties
Dark gray to black, strongly foliated and sparkly, with platy mica grains and pale quartz-rich streaks. Typical hardness is about 2.5–3 for biotite and 6–7 for quartz; mica has one perfect cleavage, while the rock has pronounced schistosity and moderate density (~2.7–3.0 g/cm³).
Formation & geological history
Formed when shale or another clay-rich sediment was subjected to elevated temperature and pressure during regional metamorphism, commonly during mountain building. Its exact geological age cannot be determined from the photograph and depends on locality.
Uses & applications
Used locally as decorative stone, landscaping material, and occasionally dimension stone; it is primarily a teaching and collector specimen rather than a jewelry material.
Geological facts
The glitter comes from aligned mica flakes reflecting light, producing the characteristic schistose texture. Dark coloration may indicate abundant biotite, graphite, amphibole, or weathered sulfides.
Field identification & locations
Common in metamorphic belts worldwide, including the Appalachians, New England, Scandinavia, and the Himalaya. Confirm by observing flakes that split along foliation; a streak test, hand lens, and locality information would help distinguish biotite schist from amphibolite or graphite schist.
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