igneous
Tumbled dark basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark brown-black, opaque, and smoothly polished with a dull to subtly vitreous luster; typically massive and nonfoliated, with Mohs hardness about 6. Specific gravity is commonly 2.8–3.0; individual crystals are too fine to see clearly.
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Physical properties
Dark brown-black, opaque, and smoothly polished with a dull to subtly vitreous luster; typically massive and nonfoliated, with Mohs hardness about 6. Specific gravity is commonly 2.8–3.0; individual crystals are too fine to see clearly.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. Basalts range from Archean to modern age; this specimen’s age cannot be determined visually.
Uses & applications
Used widely as crushed aggregate, road stone, riprap, building stone, and basalt fiber; attractive pieces are sold as tumbled stones and lapidary material. Common specimens usually have modest value.
Geological facts
Its dark color reflects abundant iron- and magnesium-rich minerals. Unlike glassy obsidian, basalt generally has a fine crystalline texture and may attract a weak magnet if magnetite is present.
Field identification & locations
Identify by its uniform dark color, fine grain, high density, and lack of visible layering or cleavage; a streak, magnet test, and fresh unpolished surface help confirm it. Common worldwide, especially Hawaii, Iceland, India, ocean floors, and volcanic regions.
