igneous
Rounded dark pebble, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; commonly basalt (plagioclase, pyroxene, ± olivine)
AI-generated identification · may be incorrectSmooth, weathered, subrounded pebble with dull dark gray-brown color and fine texture; typically Mohs 5–6, low-glassy luster, massive structure, no visible cleavage, and specific gravity about 2.7–3.1.
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Physical properties
Smooth, weathered, subrounded pebble with dull dark gray-brown color and fine texture; typically Mohs 5–6, low-glassy luster, massive structure, no visible cleavage, and specific gravity about 2.7–3.1.
Formation & geological history
Likely formed when lava cooled rapidly at or near Earth’s surface, then was rounded by stream, beach, or glacial abrasion. Exact age and composition cannot be determined from the photograph.
Uses & applications
Used as landscaping stone, aggregate, road material, and occasionally as a polishing or decorative specimen; generally little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly forms oceanic crust and large lava flows.
Field identification & locations
Identify with a hand lens as a dense, uniform, fine-grained rock; a streak test, magnetism test, hardness test, and fresh broken surface would help distinguish basalt from andesite or weathered industrial slag. Common worldwide, especially volcanic regions, river gravels, and beaches.
