igneous
Weathered dark volcanic rock, possibly basalt
Basalt (tentative; likely iron-rich mafic volcanic rock)
AI-generated identification · may be incorrectDark gray to black, fine-grained, with dull to weakly glassy luster and rusty-brown iron-oxide weathering; typically Mohs 5–6, dense, and lacking visible cleavage. The photograph is insufficient to confirm mineral grains or vesicles.
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Physical properties
Dark gray to black, fine-grained, with dull to weakly glassy luster and rusty-brown iron-oxide weathering; typically Mohs 5–6, dense, and lacking visible cleavage. The photograph is insufficient to confirm mineral grains or vesicles.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in lava flows, oceanic crust, and volcanic islands. Its exact age and locality cannot be determined visually.
Uses & applications
Used extensively as crushed aggregate, road base, riprap, railroad ballast, and building stone; attractive polished pieces may serve as decorative material. This small weathered specimen has little commercial value.
Geological facts
Typical minerals are plagioclase feldspar, pyroxene, and often olivine or magnetite. Brown patches commonly represent oxidation of iron-bearing minerals rather than a separate gemstone.
Field identification & locations
Look for a heavy, dark, fine-grained texture and black streak; a steel scratch test, magnetism test, and fresh broken surface help distinguish basalt from slag or altered rocks. Common worldwide, especially volcanic regions and oceanic settings.
