igneous
Tumbled dark stone, likely basalt or fine-grained mafic rock
Basalt (probable); commonly composed of plagioclase, pyroxene, and minor olivine or magnetite
AI-generated identification · may be incorrectHardness about 5–6 Mohs; dark gray-brown, opaque, and dull to weakly subvitreous with a smooth tumbled surface. Typically fine-grained with no visible crystals, lacking cleavage; specific gravity approximately 2.8–3.1.
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Physical properties
Hardness about 5–6 Mohs; dark gray-brown, opaque, and dull to weakly subvitreous with a smooth tumbled surface. Typically fine-grained with no visible crystals, lacking cleavage; specific gravity approximately 2.8–3.1.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust. Its age cannot be determined from the photograph and may range from recent to billions of years.
Uses & applications
Used as aggregate, road stone, building material, ballast, and landscaping rock; polished or tumbled pieces are inexpensive curios and teaching specimens.
Geological facts
The brown color may result from weathering or iron oxidation. Similar dark tumbled stones can also be andesite, chert, or slag, so identification is provisional without tests.
Field identification & locations
Look for a very fine texture, dark color, high density, and absence of obvious layering or large crystals; a streak, magnetism, hardness, and fresh broken surface would help distinguish basalt from chert or slag. Common worldwide, especially volcanic regions and river gravels.
