metamorphic

Dark gneiss or amphibolite cobble

Banded metamorphic rock, likely quartz–feldspar–biotite gneiss or amphibolite

AI-generated identification · may be incorrect

Hardness about 6–7 Mohs; dark gray to black, granular, with weak foliation and lighter quartz/feldspar bands. It has a dull to subtly sparkling luster and no obvious cleavage as a whole rock; exact identification requires a fresh surface and mineral tests.

Identified September 2, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when preexisting igneous or sedimentary rock was subjected to high temperature and pressure during regional metamorphism, commonly in Precambrian to Paleozoic continental crust. Its rounded shape indicates later stream or shoreline erosion.

Uses & applications

Used locally as decorative stone, landscaping material, aggregate, and occasionally building stone; attractive pieces may be collected but are generally low value.

Geological facts

The dark minerals are likely biotite, hornblende, or pyroxene, while pale grains are quartz and feldspar. A magnet may show weak attraction if iron-rich amphibole or magnetite is present.

Field identification & locations

Common in metamorphic terrains and glacial or river gravels; identify with visible foliation, interlocking grains, and hardness greater than glass. A streak, acid, magnet, and thin-section test would refine the diagnosis.