igneous
Likely basalt or mafic metamorphic rock
Basalt, predominantly plagioclase feldspar and pyroxene; possibly metamorphosed to amphibolite
AI-generated identification · may be incorrectDark gray to black, fine- to medium-grained, dull to slightly waxy luster, irregular fracture, and estimated Mohs hardness 5–6. Pale metallic-looking patches may be weathered feldspar, amphibole, or mica; crystal structure is typically microscopic and interlocking.
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Physical properties
Dark gray to black, fine- to medium-grained, dull to slightly waxy luster, irregular fracture, and estimated Mohs hardness 5–6. Pale metallic-looking patches may be weathered feldspar, amphibole, or mica; crystal structure is typically microscopic and interlocking.
Formation & geological history
If basalt, it formed from rapidly cooled mafic lava, commonly in oceanic crust or volcanic fields, at any geologic age. The specimen’s rough, foliated-looking surface could also indicate low- to medium-grade metamorphism of basalt.
Uses & applications
Used as crushed aggregate, road base, riprap, and building stone; attractive polished pieces may be used decoratively. Ordinary hand specimens have little commercial value.
Geological facts
Basalt is the most abundant volcanic rock and commonly contains iron- and magnesium-rich minerals. A streak, magnetism test, and close inspection for visible crystals help distinguish it from similar dark rocks.
Field identification & locations
Common worldwide, especially oceanic islands, mid-ocean ridges, and continental volcanic provinces; notable sources include Iceland, Hawaii, India, and the Columbia River region. Confirm with a fresh surface, dilute acid test, magnet, and hand lens; image-based identification is provisional.
