igneous
Weathered basalt (possible volcanic bomb)
Basalt; primarily plagioclase feldspar, clinopyroxene, and iron-titanium oxides
AI-generated identification · may be incorrectDark brown-black, fine-grained and weakly crystalline, with dull to slightly vitreous luster and irregular conchoidal-to-uneven fracture; Mohs hardness about 6, specific gravity roughly 2.8-3.1. The rusty brown surface suggests oxidation or weathering, while the rounded form may reflect transport or volcanic ejection.
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Physical properties
Dark brown-black, fine-grained and weakly crystalline, with dull to slightly vitreous luster and irregular conchoidal-to-uneven fracture; Mohs hardness about 6, specific gravity roughly 2.8-3.1. The rusty brown surface suggests oxidation or weathering, while the rounded form may reflect transport or volcanic ejection.
Formation & geological history
Basalt forms when low-viscosity mafic lava rapidly cools at or near Earth’s surface, commonly during the Cenozoic or younger volcanic activity; a bomb-like shape would indicate rapid cooling during an eruption. Exact age and origin cannot be determined from the photograph alone.
Uses & applications
Used widely as crushed aggregate, roadstone, railroad ballast, riprap, and dimension stone; attractive pieces may be collected or used decoratively, though this specimen has modest gem value.
Geological facts
Iron-bearing minerals commonly weather to reddish-brown coatings. Its apparent coarse, pale-gray crust could also represent altered basalt or a different volcanic rock, so identification is provisional without a fresh surface or testing.
Field identification & locations
Look for a heavy, dark, fine-grained rock, often with tiny feldspar crystals and no visible quartz; a streak, magnet test, hardness test, and fresh break would help confirm it. Common in volcanic regions and stream gravels.
