igneous
Likely basalt
Basalt, a mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine or iron oxides
AI-generated identification · may be incorrectDark gray to black, fine-grained and massive, with a dull to subtly vitreous luster; Mohs hardness about 5–6, no prominent cleavage, and specific gravity about 2.8–3.0. The visible elongated texture may indicate weak flow alignment or alteration.
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Physical properties
Dark gray to black, fine-grained and massive, with a dull to subtly vitreous luster; Mohs hardness about 5–6, no prominent cleavage, and specific gravity about 2.8–3.0. The visible elongated texture may indicate weak flow alignment or alteration.
Formation & geological history
Forms when low-silica, iron- and magnesium-rich lava cools rapidly at or near Earth’s surface; most examples are Cenozoic or younger, though basalt occurs throughout geological time. Common in oceanic crust, volcanic plateaus, lava flows, and dikes.
Uses & applications
Crushed basalt is used for aggregate, road base, railway ballast, concrete, and basalt fiber; sound dense pieces are also collected as volcanic specimens. It is generally not a jewelry material.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, olivine crystals, or magnetic behavior can help distinguish it from similar dark rocks.
Field identification & locations
Identify by its uniformly dark, fine-grained appearance, heavy feel, and lack of visible large crystals; a streak test is usually gray, and weak attraction to a magnet may occur. Common worldwide, especially Hawaii, Iceland, India’s Deccan Traps, and oceanic islands.
