igneous
Dark mafic rock, likely basalt
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectUsually fine-grained, dark gray to black, with dull to faintly glassy luster; Mohs hardness about 5–6, specific gravity approximately 2.8–3.0, and generally no visible cleavage. The rounded pieces appear weathered or water-worn, and some may contain vesicles or alteration.
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Physical properties
Usually fine-grained, dark gray to black, with dull to faintly glassy luster; Mohs hardness about 5–6, specific gravity approximately 2.8–3.0, and generally no visible cleavage. The rounded pieces appear weathered or water-worn, and some may contain vesicles or alteration.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Individual specimens cannot establish a precise age; basalt ranges from modern eruptions to Archean rocks billions of years old.
Uses & applications
Used extensively as crushed aggregate, road stone, railway ballast, riprap, and building stone; attractive polished pieces may be used decoratively. These small weathered fragments generally have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and forms much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its dark color, fine texture, heaviness, and resistance to scratching; a glassy streak or vesicles supports volcanic origin. Common sources include volcanic terrains, river gravels, beaches, and quarry spoil; exact identification would benefit from a fresh broken surface or thin section.
