igneous
Quartz-veined basalt or mafic schist
Basaltic rock dominated by plagioclase–pyroxene minerals, with milky quartz/calcite veins
AI-generated identification · may be incorrectDark gray to black, fine-grained and dense, with a dull to subvitreous luster and irregular fracture; the pale material is likely quartz or calcite. Basaltic portions are typically Mohs 5–6, while quartz is 7 and calcite is 3; no reliable crystal structure or specific gravity can be determined from the image.
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Physical properties
Dark gray to black, fine-grained and dense, with a dull to subvitreous luster and irregular fracture; the pale material is likely quartz or calcite. Basaltic portions are typically Mohs 5–6, while quartz is 7 and calcite is 3; no reliable crystal structure or specific gravity can be determined from the image.
Formation & geological history
Likely an ancient mafic lava or metamorphosed mafic rock cut by later silica- or carbonate-rich hydrothermal veins. Exact geological age and locality cannot be determined visually.
Uses & applications
Commonly used as aggregate, riprap, road stone, and decorative landscape rock; attractive polished pieces may be used as display specimens, but this example appears mainly of collector interest.
Geological facts
The sharp contrast between black host rock and pale vein material records fluid movement through fractures after the host rock formed. A vinegar or dilute hydrochloric-acid test can distinguish calcite (effervesces) from quartz (no reaction).
Field identification & locations
Look for a dark, heavy, fine-grained matrix containing contrasting white veins; basalt commonly occurs in volcanic terrains, while similar rocks are widespread in metamorphic belts. A hardness test and acid test are needed for confident identification.
