igneous
Polished black stone, likely basalt
Basalt; predominantly plagioclase feldspar and pyroxene, commonly with minor olivine and magnetite
AI-generated identification · may be incorrectTypically dark gray to black with a fine-grained texture, dull to subtle glassy luster, and no visible crystals; hardness about 6 Mohs, massive structure, and specific gravity roughly 2.8–3.0. The smooth rounded surface indicates polishing or tumbling.
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Physical properties
Typically dark gray to black with a fine-grained texture, dull to subtle glassy luster, and no visible crystals; hardness about 6 Mohs, massive structure, and specific gravity roughly 2.8–3.0. The smooth rounded surface indicates polishing or tumbling.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and lava flows. Most examples are Cenozoic or younger, though basalt occurs throughout geologic time.
Uses & applications
Used as crushed aggregate, road stone, building stone, paving, and decorative polished stone; generally has little gemstone value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its black color commonly reflects abundant pyroxene, magnetite, and other iron-bearing minerals.
Field identification & locations
Look for uniformly dark color, fine grain, high density, and a faint streak or magnetism from magnetite; a stronger magnet response may indicate magnetite-rich basalt. Common worldwide, especially volcanic regions and oceanic settings.
