igneous
Tumbled basalt (likely basalt–feldspar/zeolite-bearing stone)
Basalt; predominantly plagioclase feldspar and pyroxene, with possible olivine or secondary alteration minerals
AI-generated identification · may be incorrectDark gray to black, fine-grained and weakly mottled with pale alteration zones; dull to slightly vitreous luster, massive texture, no visible cleavage, and approximately 5–6 Mohs hardness. The rounded shape indicates mechanical tumbling or water wear.
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Physical properties
Dark gray to black, fine-grained and weakly mottled with pale alteration zones; dull to slightly vitreous luster, massive texture, no visible cleavage, and approximately 5–6 Mohs hardness. The rounded shape indicates mechanical tumbling or water wear.
Formation & geological history
Basalt forms when low-viscosity mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust; most examples are Cenozoic or younger, though basalt occurs throughout geologic time.
Uses & applications
Used as aggregate, roadstone, building stone, ballast, landscaping rock, and inexpensive polished décor or worry stones; it has little gemstone value.
Geological facts
Its dark color reflects iron- and magnesium-rich minerals, while pale patches may be feldspar, calcite, or weathering products. A magnet may show weak attraction if magnetite is present.
Field identification & locations
Identify by its dark, fine-grained massive appearance, relatively high density, and lack of large crystals; vinegar fizzing would suggest calcite-bearing material rather than pure basalt. Common worldwide, especially volcanic regions and river or beach gravels.
