igneous
Weathered basalt with quartz or calcite veining
Basalt (fine-grained mafic volcanic rock), locally altered and vein-filled
AI-generated identification · may be incorrectDark brown to black, fine-grained, dull to subtly glassy surface with pale cream mineral veins. Basalt typically has Mohs hardness 5–6, no visible cleavage, massive texture, and specific gravity about 2.8–3.0; the pale vein material may be quartz (hardness 7) or calcite (hardness 3, effervesces in dilute acid).
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Physical properties
Dark brown to black, fine-grained, dull to subtly glassy surface with pale cream mineral veins. Basalt typically has Mohs hardness 5–6, no visible cleavage, massive texture, and specific gravity about 2.8–3.0; the pale vein material may be quartz (hardness 7) or calcite (hardness 3, effervesces in dilute acid).
Formation & geological history
Formed when mafic lava rapidly cooled at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; later weathering and hydrothermal fluids introduced the pale veins.
Uses & applications
Used as crushed aggregate, roadstone, riprap, and building stone; attractive polished pieces are collected, though this specimen has modest gem value.
Geological facts
Brown coloration likely reflects iron-oxide weathering (limonite/goethite). A scratch test and dilute hydrochloric-acid test can distinguish quartz from calcite in the veins.
Field identification & locations
Common in volcanic regions, oceanic crust, and river gravels; identify by its dense, fine-grained dark matrix, lack of foliation, and occasional tiny feldspar crystals. Likely locations include the Pacific Northwest, Iceland, Hawaii, and many continental volcanic belts.
