igneous
Iron-rich basalt (likely weathered volcanic rock)
Basalt; chiefly plagioclase feldspar, pyroxene, and iron-rich oxides
AI-generated identification · may be incorrectTypically fine-grained, dark gray to black, with dull to submetallic luster and irregular fracture; Mohs hardness about 5–6. The rusty-brown areas likely reflect oxidation of iron-bearing minerals, and the surface appears naturally weathered or possibly polished.
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Physical properties
Typically fine-grained, dark gray to black, with dull to submetallic luster and irregular fracture; Mohs hardness about 5–6. The rusty-brown areas likely reflect oxidation of iron-bearing minerals, and the surface appears naturally weathered or possibly polished.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; its exact age cannot be determined from the photograph.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, railway ballast, and building stone; attractive polished pieces are collected but usually have modest gem value.
Geological facts
Basalt is the most abundant volcanic rock and forms much of oceanic crust, including widespread seafloor and flood-basalt provinces.
Field identification & locations
Identify by its heavy, dark, fine-grained appearance, lack of visible large crystals, and possible attraction to a strong magnet if magnetite-rich. Common worldwide in volcanic regions, including oceanic islands, mid-ocean ridges, and continental lava fields.
