metamorphic
Blue-gray gneiss (likely biotite-bearing)
Gneiss; quartz–feldspar–biotite assemblage
AI-generated identification · may be incorrectTypically 6–7 Mohs; gray to blue-gray, granular, with dark biotite streaks and rusty iron-oxide staining. Displays compositional banding/foliation, vitreous quartz, and feldspar with relatively poor cleavage at the rock scale.
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Physical properties
Typically 6–7 Mohs; gray to blue-gray, granular, with dark biotite streaks and rusty iron-oxide staining. Displays compositional banding/foliation, vitreous quartz, and feldspar with relatively poor cleavage at the rock scale.
Formation & geological history
Formed by high-grade regional metamorphism of granite or other felsic protoliths, causing mineral segregation into bands; commonly Precambrian to Paleozoic in exposed terrains. The image may also represent a foliated granitic rock, so exact identification requires a fresh surface and testing.
Uses & applications
Used as dimension stone, architectural facing, paving, monuments, and aggregate; attractive polished slabs are used decoratively. Small hand specimens generally have modest collector value.
Geological facts
Gneissic banding records intense heat and pressure deep in Earth’s crust, often during mountain building. Rust-colored specks commonly reflect oxidation of iron-bearing minerals such as biotite or pyrite.
Field identification & locations
Look for alternating light quartz/feldspar and dark mica bands, visible foliation, and a hard scratch-resistant surface; a glassy quartz grain and feldspar cleavage support identification. Common in ancient continental shields and metamorphic mountain belts worldwide.
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