igneous
Dark weathered igneous cobble, likely basalt
Basalt; fine-grained mafic rock composed chiefly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectTypically dark gray to brown, dense, and fine-grained with dull to slightly vitreous luster; Mohs hardness about 5–6, no obvious cleavage, and specific gravity roughly 2.8–3.0. The rounded shape and glossy surface appear consistent with water-worn weathering, though the photograph cannot confirm texture or composition.
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Physical properties
Typically dark gray to brown, dense, and fine-grained with dull to slightly vitreous luster; Mohs hardness about 5–6, no obvious cleavage, and specific gravity roughly 2.8–3.0. The rounded shape and glossy surface appear consistent with water-worn weathering, though the photograph cannot confirm texture or composition.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and continental lava flows. Individual flows range from recent to billions of years old; this specimen’s age and locality are undetermined.
Uses & applications
Crushed basalt is used for road aggregate, concrete, rail ballast, and construction stone; attractive pieces may be used as landscaping or display specimens rather than jewelry.
Geological facts
Field identification is supported by high density, dark color, and a very fine-grained texture; a fresh broken surface should be gray-black and may show tiny crystals or vesicles. Magnetic response can occur if magnetite is present, but similar dark rocks include andesite, diabase, and weathered ironstone.
Field identification & locations
Common worldwide, especially Hawaii, Iceland, India’s Deccan Traps, oceanic islands, and volcanic regions. Collector value is usually low unless the specimen has unusual texture, crystals, or documented provenance.
