igneous
Porphyritic basalt (likely olivine-bearing basalt)
Basalt; fine-grained mafic rock composed mainly of plagioclase and pyroxene, with possible olivine phenocrysts
AI-generated identification · may be incorrectDark gray, dense, fine-grained groundmass with abundant pale plagioclase crystals and scattered darker/brownish olivine grains; typically ~5–6 Mohs, dull to subtly vitreous luster, negligible cleavage, and specific gravity ~2.8–3.1.
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Physical properties
Dark gray, dense, fine-grained groundmass with abundant pale plagioclase crystals and scattered darker/brownish olivine grains; typically ~5–6 Mohs, dull to subtly vitreous luster, negligible cleavage, and specific gravity ~2.8–3.1.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, preserving larger crystals formed earlier in magma; commonly Cenozoic to modern in volcanic regions, though basalt occurs throughout geologic time.
Uses & applications
Used as crushed aggregate, road base, riprap, building stone, and occasionally as decorative or lapidary material; usually modest collector value.
Geological facts
The visible white crystals are most likely plagioclase phenocrysts, producing a porphyritic texture. A magnet may show weak attraction if the rock contains abundant magnetite.
Field identification & locations
Identify it by its dark mafic matrix, high density, scattered light plagioclase crystals, and lack of layering or strong foliation; common near volcanoes, oceanic crust, Hawaii, Iceland, and continental flood-basalt provinces.
