igneous
Likely tumbled basalt
Basalt, a fine-grained mafic volcanic rock; commonly composed of plagioclase, pyroxene, and minor olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, with a wet vitreous-to-dull luster; typically massive and non-cleaved. Mohs hardness is about 5–6, and specific gravity is approximately 2.8–3.0; the image’s rounded, polished shape suggests tumbling or water wear.
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Physical properties
Dark gray to black, fine-grained, with a wet vitreous-to-dull luster; typically massive and non-cleaved. Mohs hardness is about 5–6, and specific gravity is approximately 2.8–3.0; the image’s rounded, polished shape suggests tumbling or water wear.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and continental lava flows. Specimen age cannot be determined from the image and may range from recent to billions of years.
Uses & applications
Used as crushed aggregate, road stone, building stone, and basalt fiber; attractive polished pieces are used for décor and inexpensive lapidary items.
Geological facts
Often weakly magnetic if it contains magnetite. Vesicles, tiny pale plagioclase crystals, or green olivine may occur, though none are clearly visible here.
Field identification & locations
Identify by its uniformly dark fine texture, high density, and lack of obvious layering or large crystals; distinguish it from slate by its massive texture and from glass by its granular surface. Common worldwide, especially volcanic regions and river gravels.
