igneous
Dark mafic rock fragments, likely basalt
Basalt; predominantly plagioclase feldspar, clinopyroxene, and commonly olivine or iron-titanium oxides
AI-generated identification · may be incorrectTypically fine-grained, black to dark gray, with dull to slightly glassy luster; Mohs hardness about 5–6, no prominent cleavage, and specific gravity roughly 2.8–3.1. The pictured pieces appear irregular and weathered, though the image cannot confirm mineral texture or composition.
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Physical properties
Typically fine-grained, black to dark gray, with dull to slightly glassy luster; Mohs hardness about 5–6, no prominent cleavage, and specific gravity roughly 2.8–3.1. The pictured pieces appear irregular and weathered, though the image cannot confirm mineral texture or composition.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in lava flows, volcanic islands, and oceanic crust. Specimen age depends on locality, ranging from modern eruptions to billions of years.
Uses & applications
Crushed basalt is widely used as aggregate, road base, riprap, railway ballast, and basalt fiber; attractive polished pieces may serve as decorative stone. These small rough fragments have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, if present, indicate gas-rich lava; metallic black grains may be magnetite.
Field identification & locations
Identify it by its black fine-grained appearance, heavy feel, and possible weak attraction to a magnet; a streak test, hand lens, and hardness test help distinguish it from slag or coal. Common sources include Hawaii, Iceland, India, the western United States, and oceanic volcanic terrains.
