igneous
Tumbled basalt
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine and iron oxides
AI-generated identification · may be incorrectTypically Mohs 5–6, dark gray to black, with a dull to weakly vitreous luster; fine-grained, massive, and lacking visible cleavage. The rounded, polished surface and small pale mineral flecks fit tumbled basalt.
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Physical properties
Typically Mohs 5–6, dark gray to black, with a dull to weakly vitreous luster; fine-grained, massive, and lacking visible cleavage. The rounded, polished surface and small pale mineral flecks fit tumbled basalt.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands; specimens may range from Precambrian to recent. The image shows a human-tumbled and polished piece.
Uses & applications
Used widely as crushed aggregate, road material, dimension stone, and landscaping rock; polished pieces are sold as inexpensive décor, worry stones, and metaphysical collectibles.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up much of the ocean floor. Magnetic iron minerals may give some pieces a noticeable response to a strong magnet.
Field identification & locations
Identify it by its black, heavy, fine-grained appearance, lack of obvious individual crystals, and possible magnetism; a streak test is usually gray to black. Common sources include Hawaii, Iceland, India, the western United States, and oceanic volcanic regions.
