igneous
Rounded dark-gray pebble, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; commonly plagioclase ± pyroxene, amphibole, and minor olivine
AI-generated identification · may be incorrectGray-green, dull to weakly subvitreous luster, fine-grained and massive, with no visible cleavage; typical hardness about 5–6 Mohs and specific gravity roughly 2.7–3.1. The smooth shape indicates water or abrasion wear.
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Physical properties
Gray-green, dull to weakly subvitreous luster, fine-grained and massive, with no visible cleavage; typical hardness about 5–6 Mohs and specific gravity roughly 2.7–3.1. The smooth shape indicates water or abrasion wear.
Formation & geological history
Formed when lava cooled rapidly at or near Earth’s surface, generally from Cenozoic or older volcanic activity; the image alone cannot establish an exact age or whether it is basalt versus andesite.
Uses & applications
Used as aggregate, road base, riprap, landscaping stone, and occasionally as a decorative or teaching specimen; normally little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly forms oceanic crust and lava flows. Andesite is compositionally intermediate and is frequent in volcanic arcs.
Field identification & locations
Look for a uniformly fine texture, dark gray color, high density, and lack of layering; a streak, magnetism test, and thin-section or chemical analysis can distinguish basalt from andesite. Common in volcanic regions and stream gravels.
