igneous
Polished dark gray stone, likely basalt or andesite
Fine-grained mafic volcanic rock, commonly basalt (plagioclase ± pyroxene, olivine, and iron oxides)
AI-generated identification · may be incorrectHardness about 5.5–6.5 Mohs; smooth tumbled surface, gray to dark gray color, dull to faintly satin luster, and very fine-grained interlocking crystal structure. No visible cleavage or large crystals are apparent.
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Physical properties
Hardness about 5.5–6.5 Mohs; smooth tumbled surface, gray to dark gray color, dull to faintly satin luster, and very fine-grained interlocking crystal structure. No visible cleavage or large crystals are apparent.
Formation & geological history
Likely formed from rapidly cooled lava, generally in volcanic settings from the Precambrian to Recent. The rounded shape and sheen indicate human tumbling or polishing rather than natural weathering alone.
Uses & applications
Used as a decorative tumbled stone, massage stone, landscaping material, aggregate, road metal, and building stone; usually little jewelry value.
Geological facts
Basalt is the most widespread volcanic rock on Earth and forms much of the oceanic crust. Its dark color reflects abundant iron- and magnesium-bearing minerals.
Field identification & locations
Identify basalt by its substantial weight, fine texture, lack of visible grains, and inability to scratch glass only if sufficiently hard; a streak, magnetism, and acid test can help distinguish it from slag or other stones. Common worldwide, especially volcanic regions.
