igneous
Rounded basalt cobble
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to slightly subvitreous luster, with no visible cleavage; hardness about 6 on the Mohs scale and specific gravity roughly 2.8–3.0. Its smooth shape reflects abrasion rather than crystal form.
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-brown, fine-grained, dull to slightly subvitreous luster, with no visible cleavage; hardness about 6 on the Mohs scale and specific gravity roughly 2.8–3.0. Its smooth shape reflects abrasion rather than crystal form.
Formation & geological history
An extrusive igneous rock formed when low-silica lava rapidly cooled at or near Earth’s surface, generally from the Cenozoic or older volcanic activity. The rounded cobble was later transported and polished by a river, shoreline, or glacial environment.
Uses & applications
Used as aggregate, road base, riprap, paving stone, and landscaping material; attractive pieces may be polished or collected, but this specimen has little gem value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, dikes, and ocean-floor pillow lavas.
Field identification & locations
Identify it by its heavy, dark, compact, fine-grained texture and lack of obvious sedimentary layers; a streak test is usually gray, and it generally does not fizz with dilute hydrochloric acid. Common worldwide, especially volcanic regions and river gravels.
