metamorphic

Rounded dark metamorphic rock, likely amphibolite

Amphibolite; chiefly hornblende amphibole and plagioclase feldspar

AI-generated identification · may be incorrect

Typically dark green-black, fine- to medium-grained, with a dull to weakly glassy luster and massive or faintly foliated texture. Hardness is about 5–6; cleavage is poor overall, though amphibole grains have two cleavages, and specific gravity is roughly 2.9–3.2.

Identified September 10, 2026More metamorphic →

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Physical properties

Formation & geological history

Forms when basaltic or gabbroic protoliths undergo medium- to high-grade regional metamorphism, commonly during mountain building. Most examples are Precambrian to Phanerozoic, though the specimen’s age cannot be determined visually.

Uses & applications

Used locally as crushed aggregate, road stone, riprap, and dimension stone; attractive polished pieces are collected but are uncommon jewelry material.

Geological facts

The rounded shape indicates water or glacial transport, producing a cobble rather than a freshly exposed outcrop fragment.

Field identification & locations

Look for a heavy dark-green/black rock with visible amphibole needles or subtle foliation; it may show white plagioclase flecks and does not fizz with dilute acid. Common in metamorphic belts and glacial deposits.