igneous
Polished basalt pebble
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectFine-grained, dark gray to black with a subdued glassy polish; typically hardness 5–6 Mohs, massive texture, no visible cleavage, and specific gravity about 2.8–3.0.
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Physical properties
Fine-grained, dark gray to black with a subdued glassy polish; typically hardness 5–6 Mohs, massive texture, no visible cleavage, and specific gravity about 2.8–3.0.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust; its age may range from recent to billions of years. The rounded shape and shine indicate water-worn and/or polished processing.
Uses & applications
Used as aggregate, road stone, paving, monuments, landscaping, and decorative polished stones; generally low jewelry value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the ocean floor. Small dark pebbles may also be fine-grained andesite or another mafic volcanic rock, which cannot be confirmed from the image alone.
Field identification & locations
Identify it by its uniformly dark color, fine grain, heavy feel, and lack of obvious crystals or banding; common sources include volcanic regions, river gravels, beaches, and manufactured stone shops.
