igneous
Polished basalt (likely vesicular basalt)
Basalt; predominantly plagioclase, clinopyroxene, and commonly olivine or iron-titanium oxides
AI-generated identification · may be incorrectDark gray to black, fine-grained, with a dull to glassy polish and possible small vesicles; hardness about 5–6 Mohs, massive texture, negligible cleavage, and specific gravity roughly 2.8–3.1. The pale reflective patches may be polish, weathering, or mineral-filled cavities rather than a separate rock.
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Physical properties
Dark gray to black, fine-grained, with a dull to glassy polish and possible small vesicles; hardness about 5–6 Mohs, massive texture, negligible cleavage, and specific gravity roughly 2.8–3.1. The pale reflective patches may be polish, weathering, or mineral-filled cavities rather than a separate rock.
Formation & geological history
An extrusive volcanic rock formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic to present-day volcanism. Typical settings include shield volcanoes, lava flows, and oceanic crust.
Uses & applications
Used as crushed aggregate, road stone, dimension stone, countertops, and decorative polished specimens; it has limited jewelry use except as carvings or cabochons. Common pieces are inexpensive.
Geological facts
Basalt is the most abundant volcanic rock and forms much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and relatively low silica content.
Field identification & locations
Identify it by its black, dense, fine-grained appearance, lack of visible large crystals, and strong response only if magnetite is abundant; a hand lens may reveal tiny plagioclase crystals or vesicles. Common worldwide, especially Hawaii, Iceland, India’s Deccan Traps, and oceanic islands.
