igneous
Tumbled dark mafic rock, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or iron-titanium oxides
AI-generated identification · may be incorrectFine-grained, dark gray-green to black, with a dull to weakly waxy luster and no visible cleavage; typical hardness is about 6 on the Mohs scale and specific gravity roughly 2.8–3.0. The rounded, glossy surface indicates water-worn or tumbled basalt.
Identified More igneous →
Explore Tumbled dark mafic rock, likely basalt in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Fine-grained, dark gray-green to black, with a dull to weakly waxy luster and no visible cleavage; typical hardness is about 6 on the Mohs scale and specific gravity roughly 2.8–3.0. The rounded, glossy surface indicates water-worn or tumbled basalt.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Individual pieces may range from modern lava to rocks hundreds of millions of years old; age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road stone, riprap, ballast, building stone, and decorative landscaping material; polished pieces are inexpensive curios or lapidary items.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Small pale specks may be feldspar, while dark metallic grains can be magnetite or ilmenite.
Field identification & locations
Field clues include high density, dark fine-grained texture, blocky fracture, and sometimes attraction to a strong magnet; acid produces no reaction unless weathered carbonate is present. Common worldwide, especially volcanic regions and stream gravels.
