igneous
Porphyritic basalt (likely basaltic breccia or conglomeratic basalt)
Basalt, chiefly plagioclase ± pyroxene/olivine in a fine-grained mafic matrix
AI-generated identification · may be incorrectDark gray to black, fine-grained, with scattered pale angular-to-rounded mineral or rock fragments; typically dull to weakly vitreous, Mohs hardness about 5–6, no obvious cleavage, and specific gravity about 2.8–3.1. The image alone cannot confirm whether the light pieces are feldspar phenocrysts or inclusions.
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Physical properties
Dark gray to black, fine-grained, with scattered pale angular-to-rounded mineral or rock fragments; typically dull to weakly vitreous, Mohs hardness about 5–6, no obvious cleavage, and specific gravity about 2.8–3.1. The image alone cannot confirm whether the light pieces are feldspar phenocrysts or inclusions.
Formation & geological history
Forms when mafic lava cools rapidly at or near Earth’s surface; the irregular pale fragments may indicate brecciation or incorporation of older material. Basalt ranges from Precambrian to recent and occurs in volcanic provinces, oceanic crust, and lava flows.
Uses & applications
Commonly used as aggregate, road stone, riprap, and building stone; attractive polished pieces may be sold as cabinet specimens, but this example appears to have modest commercial value.
Geological facts
Basalt is the most widespread volcanic rock on Earth, composing much of the ocean floor. Pale plagioclase crystals are common in basalt, while rounded fragments suggest transport or weathering before cementation.
Field identification & locations
Identify by its high density, dark mafic groundmass, and hardness; a streak, fresh fracture, and hand lens would help distinguish basalt from slag or a breccia. Common in volcanic regions worldwide, including Hawaii, Iceland, India, and the Pacific Northwest.
