igneous
Dark rounded stone, likely basalt or weathered andesite
Fine-grained mafic volcanic rock; commonly plagioclase and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectGray-black, dull to weakly waxy luster, massive and fine-grained; estimated Mohs hardness 5–6, no obvious cleavage, and specific gravity about 2.8–3.1. The rounded form suggests water or abrasion weathering; exact identification requires a fresh surface or testing.
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Physical properties
Gray-black, dull to weakly waxy luster, massive and fine-grained; estimated Mohs hardness 5–6, no obvious cleavage, and specific gravity about 2.8–3.1. The rounded form suggests water or abrasion weathering; exact identification requires a fresh surface or testing.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, generally in volcanic settings from the Precambrian to Recent. The specimen was later rounded by transport or abrasion.
Uses & applications
Basalt is used as aggregate, road stone, riprap, building stone, and occasionally for polished decorative objects. This small unverified specimen has minimal commercial value.
Geological facts
Basalt commonly contains microscopic crystals, so its texture may appear uniformly dark without visible grains. A magnetic response can occur because of magnetite.
Field identification & locations
Look for a dense, dark, fine-grained rock with tiny pale plagioclase specks and possible magnetism; a streak, hardness, density, and acid test can help distinguish it from slag or other stones. Common worldwide, especially near volcanic provinces and oceanic crust.
