igneous
Weathered basaltic rock (likely basalt)
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-green, dull to slightly earthy luster, fine-grained and massive, with no visible crystals or cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1. The brown patches are likely iron-oxide weathering.
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Physical properties
Dark gray-green, dull to slightly earthy luster, fine-grained and massive, with no visible crystals or cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1. The brown patches are likely iron-oxide weathering.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and ancient lava flows. Its exact geological age cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and construction stone; unremarkable rounded pieces generally have little jewelry or collector value.
Geological facts
Basalt is the most common volcanic rock on Earth and forms much of the oceanic crust. Weathering can produce greenish surfaces from altered iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its heavy, fine-grained, dark appearance, lack of layering, and possible weak attraction to a magnet; a streak, density test, and fresh broken surface improve confidence. Common worldwide, especially near lava flows and volcanic regions.
