igneous
Biotite-rich granite or granitic gneiss
Quartz–feldspar–biotite granitoid; possibly foliated granite/gneiss
AI-generated identification · may be incorrectDark gray, coarse-grained, with reflective black biotite mica, pale feldspar, and likely quartz. Mica has perfect basal cleavage; overall hardness about 6–7 Mohs, vitreous to pearly luster, and irregular to blocky fracture. Exact identification is limited because the specimen is weathered and lacks a fresh surface.
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Physical properties
Dark gray, coarse-grained, with reflective black biotite mica, pale feldspar, and likely quartz. Mica has perfect basal cleavage; overall hardness about 6–7 Mohs, vitreous to pearly luster, and irregular to blocky fracture. Exact identification is limited because the specimen is weathered and lacks a fresh surface.
Formation & geological history
Likely crystallized slowly from silica-rich magma deep in the crust, or represents metamorphosed granite if the minerals show consistent banding. Such rocks are generally Precambrian or younger basement material; a precise age cannot be determined visually.
Uses & applications
Used as crushed stone, aggregate, building stone, and occasionally decorative material. Unlikely to have significant gemstone value unless it contains unusually attractive feldspar, quartz, or mica textures.
Geological facts
The prominent shiny flakes are probably biotite rather than metallic ore. A magnet test may show little or weak attraction if magnetite is present.
Field identification & locations
Commonly found in crystalline basement terrains, including parts of central Africa near the reported location. Look for interlocking grains, pale feldspar, gray translucent quartz, and mica flakes; a fresh chip and hand lens would distinguish granite from gneiss or schist. Value is typically $5–$30 for a hand specimen, or less as ordinary local rock.
