igneous
Likely dark volcanic rock (basalt or andesite)
Fine-grained mafic to intermediate volcanic rock; commonly plagioclase ± pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectDark gray-green, dull to weakly vitreous, apparently fine-grained and massive; typical hardness about 5–6 Mohs, no visible cleavage, and specific gravity roughly 2.7–3.1. The photograph is insufficient to distinguish basalt from andesite confidently.
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Physical properties
Dark gray-green, dull to weakly vitreous, apparently fine-grained and massive; typical hardness about 5–6 Mohs, no visible cleavage, and specific gravity roughly 2.7–3.1. The photograph is insufficient to distinguish basalt from andesite confidently.
Formation & geological history
Formed when lava cooled rapidly at or near Earth’s surface, generally during the Cenozoic or older volcanic episodes. Common in lava flows, volcanic islands, oceanic crust, and shield volcanoes.
Uses & applications
Basalt is widely used as crushed aggregate, road stone, riprap, and occasionally dimension stone; andesite has similar construction uses. This small unverified piece has little commercial or collector value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the oceanic crust. Green coloration may reflect weathering, chlorite, or olivine alteration rather than a separate mineral species.
Field identification & locations
Look for a very fine texture, heavy feel, dark color, and absence of bubbles; a streak test is usually pale gray. Freshly broken surfaces, a magnet test, hardness testing, and a loupe or thin section would improve identification; common worldwide, especially volcanic regions.
