igneous
Tumbled black basalt
Basalt; fine-grained plagioclase–pyroxene volcanic rock, commonly with olivine and iron oxides
AI-generated identification · may be incorrectTypically Mohs 5–6, opaque black to dark gray, with dull to weakly glassy luster and very fine interlocking crystals. The rounded, polished form and bluish reflective patches may indicate weathering, vesicle filling, or surface coating; exact identification cannot be confirmed visually.
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Physical properties
Typically Mohs 5–6, opaque black to dark gray, with dull to weakly glassy luster and very fine interlocking crystals. The rounded, polished form and bluish reflective patches may indicate weathering, vesicle filling, or surface coating; exact identification cannot be confirmed visually.
Formation & geological history
Forms when mafic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic plateaus, and lava flows. Basalt ranges from Precambrian to modern and is actively forming today.
Uses & applications
Used as crushed aggregate, roadstone, building stone, and landscaping material; polished pieces are sold as decorative stones and occasionally marketed as grounding or healing stones.
Geological facts
Basalt is the most widespread volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Look for high density, fine grain, dark color, and weak attraction to a magnet if magnetite-rich; a glassy streak or coating can complicate testing. Common sources include Hawaii, Iceland, India, the Pacific Northwest, and oceanic islands.
