igneous
Polished basalt cobble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectTypically dark gray to reddish-brown, fine-grained, massive, and dull to subtly vitreous; Mohs hardness about 5–6, with no visible cleavage and specific gravity roughly 2.8–3.0. The smooth surface appears naturally weathered or polished, and the small pale spots may be feldspar or weathering products.
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Physical properties
Typically dark gray to reddish-brown, fine-grained, massive, and dull to subtly vitreous; Mohs hardness about 5–6, with no visible cleavage and specific gravity roughly 2.8–3.0. The smooth surface appears naturally weathered or polished, and the small pale spots may be feldspar or weathering products.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and ancient lava flows. Its age cannot be determined from the image and may range from recent to billions of years.
Uses & applications
Used as crushed aggregate, roadstone, building stone, paving, and landscaping; polished pieces are decorative or collectible but generally low value.
Geological facts
Basalt is the most abundant volcanic rock and commonly forms ocean-floor crust. Reddish coloration usually reflects oxidation of iron-bearing minerals during weathering.
Field identification & locations
Identify it by its heavy feel, fine interlocking texture, dark mafic color, and lack of layering or obvious large crystals; it should not fizz with dilute acid. Common worldwide, especially near volcanic provinces and river or beach cobble deposits.
