igneous
Tumbled dark stone, likely basalt or basaltic rock
Fine-grained mafic volcanic rock; commonly basalt, composed chiefly of plagioclase and pyroxene with possible olivine and iron oxides
AI-generated identification · may be incorrectDark gray to black, fine-grained, subdued vitreous to dull luster, and approximately 5–6 Mohs hardness. The rounded shape is abrasion-produced; visible pale veins or specks may represent quartz, calcite, feldspar, or weathering products.
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Physical properties
Dark gray to black, fine-grained, subdued vitreous to dull luster, and approximately 5–6 Mohs hardness. The rounded shape is abrasion-produced; visible pale veins or specks may represent quartz, calcite, feldspar, or weathering products.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and river gravels. Its exact geological age cannot be determined from the photograph.
Uses & applications
Used as aggregate, road stone, riprap, landscaping material, and occasionally as a polished or tumbled decorative stone. This small specimen has modest collector value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the oceanic crust. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Likely identification requires a fresh surface, hand lens, and tests: basalt is dense, nonmetallic, usually nonmagnetic or weakly magnetic, and lacks visible large crystals. Common sources include volcanic regions and streambeds; exact identification is uncertain without testing.
