igneous
Rounded gray pebble, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; commonly basalt (plagioclase ± pyroxene/olivine)
AI-generated identification · may be incorrectGray-green, fine-grained, dull to faintly satin luster, massive with no visible crystals or cleavage; estimated Mohs hardness 5–6 and specific gravity about 2.7–3.1. Its smooth triangular form appears naturally water-worn or tumbled.
Identified More igneous →
Explore Rounded gray pebble, likely basalt or andesite in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Gray-green, fine-grained, dull to faintly satin luster, massive with no visible crystals or cleavage; estimated Mohs hardness 5–6 and specific gravity about 2.7–3.1. Its smooth triangular form appears naturally water-worn or tumbled.
Formation & geological history
Likely formed when lava cooled rapidly at or near Earth’s surface, producing a fine texture, then was rounded by erosion or tumbling. Exact geological age and locality cannot be determined from the photograph.
Uses & applications
Common basalt is used as aggregate, roadstone, paving, and construction material; small attractive pieces are sold as curios or cabochon rough. Typical value is low: about $1–$10 for an ordinary specimen, or $10–$30 if locally distinctive or polished.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Andesite is a visually similar intermediate volcanic rock and cannot be confidently separated here without testing.
Field identification & locations
Look for a dense, fine-grained gray rock with scattered tiny crystals, no layering, and resistance to scratching by steel. Common sources include river gravels, beaches, volcanic regions, and landscaping stone; a streak, magnetism, acid reaction, and magnification can refine identification.
