igneous
Biotite-rich granite or granitic gneiss
Coarse-grained quartz–feldspar–biotite rock; likely granite, possibly weakly foliated gneiss
AI-generated identification · may be incorrectDark gray, coarse crystalline texture with reflective black biotite mica, pale feldspar, and likely quartz; vitreous to pearly luster. Hardness about 6–7 overall; mica shows basal cleavage, while quartz and feldspar are harder and lack obvious cleavage. Specific gravity typically 2.6–2.8.
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Physical properties
Dark gray, coarse crystalline texture with reflective black biotite mica, pale feldspar, and likely quartz; vitreous to pearly luster. Hardness about 6–7 overall; mica shows basal cleavage, while quartz and feldspar are harder and lack obvious cleavage. Specific gravity typically 2.6–2.8.
Formation & geological history
Granite crystallizes slowly from silica-rich magma deep in the crust, commonly during Precambrian to Phanerozoic continental mountain-building. If the minerals show directional alignment or banding, it may be metamorphic gneiss formed by high-grade alteration of granite.
Uses & applications
Used as crushed aggregate, building stone, monuments, countertops, and decorative landscaping material; attractive mica-rich pieces are collected as specimens.
Geological facts
Large interlocking crystals indicate slow cooling underground. The strongly sparkling appearance is produced mainly by abundant biotite mica rather than metallic minerals.
Field identification & locations
Identify it by visible quartz, feldspar, and shiny black mica in a coarse-grained aggregate; granite is massive, whereas gneiss commonly has foliation or banding. Common worldwide, especially in exposed continental shields and mountain ranges.
