igneous
Rounded basalt cobble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectTypically Mohs 5–6, dark gray to greenish-gray, dull to subtly vitreous, and very fine-grained; the specimen is strongly water-worn and may contain small vesicles or weathered mineral spots. Cleavage is not visible in the massive rock, and specific gravity is commonly about 2.8–3.0.
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
Typically Mohs 5–6, dark gray to greenish-gray, dull to subtly vitreous, and very fine-grained; the specimen is strongly water-worn and may contain small vesicles or weathered mineral spots. Cleavage is not visible in the massive rock, and specific gravity is commonly about 2.8–3.0.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands. The rounded shape indicates prolonged abrasion in a river, beach, or glacial deposit; its exact age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and occasionally dimension stone; attractive polished pieces are collected but are generally not gemstone-grade.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Green coloration can result from altered iron-rich minerals, such as chlorite or epidote.
Field identification & locations
Identify it by its dense, fine-grained texture, dark mafic color, heavy feel, and lack of obvious quartz or large crystals; confirm with a streak, hardness, and density test. Common in volcanic regions and transported gravel beds.
