igneous
Tumbled and broken dark rock fragments, likely basalt or iron-rich volcanic rock
Basalt; commonly composed of plagioclase feldspar, pyroxene, and olivine, with possible iron-oxide alteration
AI-generated identification · may be incorrectTypically dark gray to brown-black, fine-grained, dull to mildly vitreous, and lacking visible cleavage; hardness about Mohs 5–6 and specific gravity roughly 2.8–3.2. Several pieces appear weathered, vesicular, or coated with rusty iron oxides.
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Physical properties
Typically dark gray to brown-black, fine-grained, dull to mildly vitreous, and lacking visible cleavage; hardness about Mohs 5–6 and specific gravity roughly 2.8–3.2. Several pieces appear weathered, vesicular, or coated with rusty iron oxides.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. The individual specimens cannot be assigned a geological age from the image alone.
Uses & applications
Used extensively as crushed aggregate, roadstone, railroad ballast, riprap, and building stone; attractive polished pieces may be sold as inexpensive décor or lapidary material.
Geological facts
The collection appears heterogeneous, so some fragments could instead be dark chert, slag, or other iron-rich material. A fresh gray-black interior and tiny interlocking crystals support basalt identification.
Field identification & locations
Common worldwide, especially in volcanic regions. Test a fresh surface for fine-grained texture and weak attraction to a magnet; a streak, acid, density, and thin-section test would improve identification.
