igneous
Tumbled basaltic rock, likely gabbro or basalt
Mafic igneous rock, chiefly plagioclase feldspar and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectDark gray-black, fine- to medium-grained, rounded and polished; vitreous to dull luster, Mohs hardness about 5–6, generally massive with no obvious cleavage. Brown-gold areas may be weathered olivine, iron staining, or alteration minerals.
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Physical properties
Dark gray-black, fine- to medium-grained, rounded and polished; vitreous to dull luster, Mohs hardness about 5–6, generally massive with no obvious cleavage. Brown-gold areas may be weathered olivine, iron staining, or alteration minerals.
Formation & geological history
Formed when mafic magma crystallized, probably as basaltic lava or shallow/intrusive gabbro; age cannot be determined from the photograph and may range from Precambrian to recent. The rounded shape indicates stream or beach abrasion, or tumbling.
Uses & applications
Used as aggregate, road stone, riprap, ballast, and decorative landscaping material; polished pieces are sold as inexpensive display stones. Typically has little jewelry value.
Geological facts
Basalt and gabbro have broadly similar chemistry, but basalt cools rapidly with fine crystals while gabbro cools slowly with larger visible crystals. Iron-bearing minerals commonly weather to brown or orange surfaces.
Field identification & locations
Identify by its dark mafic color, high density, granular texture, and lack of layering or vesicles; a hand lens and streak/magnet test may help, though weak magnetism is possible. Common worldwide in volcanic terrains and oceanic crust.
