igneous
Rounded basaltic cobble
Basalt, likely amygdaloidal or fine-grained mafic volcanic rock
AI-generated identification · may be incorrectDark gray-black, fine-grained, subrounded and naturally weathered; mostly dull to faintly vitreous luster. Hardness about 5.5–6.5 Mohs, composed mainly of plagioclase and pyroxene with possible olivine; cleavage is not visible at this scale.
Identified More igneous →
Explore Rounded basaltic cobble in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark gray-black, fine-grained, subrounded and naturally weathered; mostly dull to faintly vitreous luster. Hardness about 5.5–6.5 Mohs, composed mainly of plagioclase and pyroxene with possible olivine; cleavage is not visible at this scale.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, probably during the Cenozoic or older depending on locality; subsequent stream or beach abrasion produced its smooth shape.
Uses & applications
Used as aggregate, roadstone, riprap, landscaping stone, and occasionally as a hand specimen or carving material; limited jewelry value unless unusually patterned or polished.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, dikes, and oceanic islands.
Field identification & locations
Identify it by its heavy dark color, fine texture, lack of obvious layering, and possible attraction to a strong magnet from iron-bearing minerals. Common worldwide in volcanic regions and river or beach gravels.
