igneous
Tumbled basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black with a dull to faintly vitreous luster; hardness about 6 on Mohs scale, fine-grained interlocking crystals, no visible cleavage, and specific gravity around 2.8–3.0. The rounded, smooth surface indicates tumbling or water wear.
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Physical properties
Typically dark gray to black with a dull to faintly vitreous luster; hardness about 6 on Mohs scale, fine-grained interlocking crystals, no visible cleavage, and specific gravity around 2.8–3.0. The rounded, smooth surface indicates tumbling or water wear.
Formation & geological history
Forms when basaltic lava rapidly cools at or near Earth’s surface, commonly in volcanic flows and oceanic crust. Most examples are geologically young, though basalt occurs from Precambrian to present-day ages.
Uses & applications
Used as aggregate, road stone, construction blocks, ballast, and landscaping material; polished or tumbled pieces are inexpensive decorative and collecting specimens.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify by its uniformly fine texture, dark color, heavy feel, and lack of large crystals; a magnet may show weak attraction if magnetite is present. Common worldwide, especially in volcanic regions and river gravels.
