igneous
Likely basaltic rock with vesicles
Vesicular basalt, possibly amygdaloidal basalt
AI-generated identification · may be incorrectDark gray to black, fine-grained volcanic rock with small cavities and rusty-brown alteration or infilling; mainly plagioclase, pyroxene, and commonly olivine. Hardness about 5–6 Mohs, dull to faintly vitreous luster, massive texture, and no obvious cleavage.
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Physical properties
Dark gray to black, fine-grained volcanic rock with small cavities and rusty-brown alteration or infilling; mainly plagioclase, pyroxene, and commonly olivine. Hardness about 5–6 Mohs, dull to faintly vitreous luster, massive texture, and no obvious cleavage.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, trapping gas bubbles; cavities may later acquire brown iron-oxide or mineral fillings. Basalt ranges from Precambrian to modern and commonly occurs in volcanic flows and oceanic crust.
Uses & applications
Used as crushed aggregate, road material, riprap, and occasionally dimension stone; attractive polished pieces are collected but are usually low-value.
Geological facts
Vesicles distinguish gas-rich lava from most intrusive rocks; filled vesicles are called amygdules. The brown areas may represent oxidation of iron-bearing minerals rather than a separate gemstone.
Field identification & locations
Look for a dark, dense matrix, tiny holes, and fine crystals; a streak test is usually gray, and it commonly attracts weakly to a magnet if magnetite is present. Common worldwide, especially Hawaii, Iceland, India, and oceanic volcanic regions.
