igneous
Dark volcanic rock, likely basalt
Basalt; predominantly calcic plagioclase and clinopyroxene, commonly with olivine and iron-titanium oxides
AI-generated identification · may be incorrectFine-grained, dark gray to black, dull to weakly vitreous; typically massive with indistinct crystals, Mohs hardness about 5–6, no prominent cleavage, and specific gravity about 2.8–3.0. The small angular fragment appears weathered and may contain vesicles or crystalline patches.
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Physical properties
Fine-grained, dark gray to black, dull to weakly vitreous; typically massive with indistinct crystals, Mohs hardness about 5–6, no prominent cleavage, and specific gravity about 2.8–3.0. The small angular fragment appears weathered and may contain vesicles or crystalline patches.
Formation & geological history
Forms when basaltic lava rapidly cools at or near Earth’s surface, especially at oceanic ridges, volcanic islands, and continental flood-basalt provinces. Most examples range from recent to Precambrian age; the specimen’s age cannot be determined visually.
Uses & applications
Widely used as crushed aggregate, roadstone, railway ballast, riprap, and building stone; attractive polished basalt is used decoratively. Ordinary fragments have little collector value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of Earth’s oceanic crust. It commonly weathers to dark brown or reddish surfaces as iron-bearing minerals oxidize.
Field identification & locations
Identify it by its uniformly dark color, fine texture, heavy feel, and lack of visible large crystals; a streak test is usually gray, and dilute acid should not fizz. Common worldwide, especially volcanic terrains; this small unprovenanced piece is typically worth $1–$10.
