igneous
Polished black pebble, likely basalt
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray to black, smooth and weakly waxy/vitreous when polished, with no visible crystals; typically Mohs 6–7, massive, with poor cleavage and specific gravity about 2.8–3.0. The image alone cannot exclude black chert, slate, or industrial glass.
Identified More igneous →
Explore Polished black pebble, likely basalt in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark gray to black, smooth and weakly waxy/vitreous when polished, with no visible crystals; typically Mohs 6–7, massive, with poor cleavage and specific gravity about 2.8–3.0. The image alone cannot exclude black chert, slate, or industrial glass.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, flood-basalt provinces, and volcanic islands. Individual specimens may be geologically young or millions to billions of years old.
Uses & applications
Used as crushed aggregate, roadstone, riprap, building stone, and occasionally as polished decorative or landscaping material; generally not a gemstone.
Geological facts
A magnet may show weak attraction if the basalt contains abundant magnetite. A glassy surface, bubbles, or conchoidal fractures would instead suggest man-made glass or natural obsidian.
Field identification & locations
Common worldwide, especially in volcanic regions and river gravels; identify by its dense black color, fine texture, and lack of layering or obvious glassy translucence. A hardness, streak, density, and acid test would improve confidence.
