igneous
Dark igneous rock, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and possibly olivine
AI-generated identification · may be incorrectFine-grained, dark gray to black, dull to weakly glassy luster, typically massive with no visible cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. The blue-gray edge may reflect lighting or a thin weathered surface.
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Physical properties
Fine-grained, dark gray to black, dull to weakly glassy luster, typically massive with no visible cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. The blue-gray edge may reflect lighting or a thin weathered surface.
Formation & geological history
Basalt forms when low-viscosity mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its age cannot be determined from the image; basalt ranges from recent lava to rocks billions of years old.
Uses & applications
Used as crushed aggregate, road metal, railroad ballast, building stone, and sometimes decorative material; this small specimen has little jewelry value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the ocean floor. It commonly contains microscopic crystals, so individual minerals may be difficult to see.
Field identification & locations
Identify it by its uniform dark color, heavy feel, fine texture, and lack of obvious layering or large crystals; a streak test is usually gray, and it should not fizz with dilute acid. Common worldwide, especially near volcanoes and oceanic crust.
