igneous
Rounded basalt cobble
Basalt, a fine-grained mafic volcanic rock; mainly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray to brown, fine-grained, dull to weakly subvitreous, with no visible cleavage; Mohs hardness about 6, specific gravity roughly 2.8–3.0. Its smooth shape and lighter brown weathering rind indicate prolonged abrasion and surface alteration.
Identified More igneous →
Explore Rounded basalt cobble in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark gray to brown, fine-grained, dull to weakly subvitreous, with no visible cleavage; Mohs hardness about 6, specific gravity roughly 2.8–3.0. Its smooth shape and lighter brown weathering rind indicate prolonged abrasion and surface alteration.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, flood-basalt provinces, and volcanic islands. The specimen is likely a river-, beach-, or glacially rounded cobble; its precise age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, and decorative landscaping stone; attractive polished pieces are collected but are generally not jewelry gemstones.
Geological facts
The rusty brown coloration commonly comes from oxidation of iron-bearing minerals, while the dark fresh interior reflects basalt’s mafic composition.
Field identification & locations
Identify by its heavy, fine-grained dark body, lack of obvious crystals, and resistance to scratching with a steel blade; distinguish it from shale or slag by its dense, natural texture and absence of glassy bubbles. Common worldwide near volcanic terrains and transported in river gravels and beaches.
