igneous
Porphyritic basalt (likely olivine- or pyroxene-bearing basalt)
Basalt; fine-grained mafic volcanic rock, commonly containing clinopyroxene and/or olivine phenocrysts
AI-generated identification · may be incorrectDark gray, fine-grained groundmass with scattered black to dark-green angular crystals; typically glassy to dull luster, no obvious cleavage, hardness about 6, and specific gravity roughly 2.8–3.1. The rounded surface appears naturally water-worn.
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Physical properties
Dark gray, fine-grained groundmass with scattered black to dark-green angular crystals; typically glassy to dull luster, no obvious cleavage, hardness about 6, and specific gravity roughly 2.8–3.1. The rounded surface appears naturally water-worn.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, producing a fine matrix around larger crystals. Basalt is generally Cenozoic or older; exact age cannot be determined visually. It is plausible near Spokane, Washington, where Columbia River Basalt and related volcanic rocks occur.
Uses & applications
Used as aggregate, road material, riprap, and building stone; attractive polished pieces are collected but usually have modest value.
Geological facts
The dark inclusions may be pyroxene or olivine, though a close photograph cannot distinguish them confidently. It is not obviously a meteorite: the texture and abundant crystal-like inclusions are more consistent with terrestrial basalt.
Field identification & locations
Identify with a hand lens, streak, and magnet test: basalt commonly has tiny interlocking crystals and may be weakly magnetic; a fresh broken surface should remain dark and granular. Common around the Columbia Plateau and Spokane-area volcanic terrains.
