metamorphic

Dark gneiss or hornblende-bearing metamorphic rock

Biotite–hornblende gneiss/amphibolite, likely quartz- and feldspar-bearing

AI-generated identification · may be incorrect

Coarse, dark brown-black specimen with visible sparkly mica and pale mineral grains; likely Mohs hardness 5–6, vitreous to submetallic luster, massive to weakly foliated texture, and no obvious cleavage at this scale. Exact identification is limited by the rounded, weathered surface.

Identified September 4, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when a basaltic or clay-rich protolith was subjected to high temperature and pressure during regional metamorphism, commonly during Precambrian to Paleozoic mountain building. Rounded edges indicate later erosion and transport as a pebble or cobble.

Uses & applications

Used locally as aggregate, riprap, landscaping stone, and occasionally decorative material; generally low jewelry value unless cut to reveal attractive banding or crystals.

Geological facts

Dark amphibole and biotite commonly give this rock its brown-black appearance, while quartz and feldspar produce the lighter flecks. A streak, hardness, and fresh broken surface would distinguish amphibolite from biotite gneiss more reliably.

Field identification & locations

Common in metamorphic terrains and glacial or river gravels. Look for aligned mica/amphibole grains, irregular light bands, and resistance to scratching; avoid confusing it with dark granite or slag.