igneous
Black basalt (likely vesicular basalt)
Basalt; chiefly plagioclase feldspar, clinopyroxene, and often olivine, with minor magnetite (approx. 45–52% SiO₂)
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to weakly glassy luster, irregular fracture, and typically Mohs hardness 6–7. The specimen appears dense and rounded, with possible weathering or small vesicles; crystal grains are not clearly visible in the image.
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Physical properties
Dark gray to black, fine-grained, dull to weakly glassy luster, irregular fracture, and typically Mohs hardness 6–7. The specimen appears dense and rounded, with possible weathering or small vesicles; crystal grains are not clearly visible in the image.
Formation & geological history
Basalt forms when low-viscosity mafic lava cools rapidly at or near Earth’s surface. It is generally Cenozoic or younger in volcanic regions; the Manila area is near the active volcanic arc and can receive basaltic-andesite material from nearby Philippine volcanoes.
Uses & applications
Commonly used as crushed aggregate, road stone, riprap, paving stone, and occasionally as a decorative or polished stone. Ordinary hand specimens have little commercial value.
Geological facts
Its dark color comes from iron- and magnesium-rich minerals. Magnetite may make it weakly magnetic, and basalt commonly produces a dark streak or gray powder when scratched.
Field identification & locations
Likely regional material around 14.6°N, 121.0°E, though river cobbles or construction fill are also possible. Test with a steel file, inspect a fresh broken surface for tiny interlocking crystals, and check for weak magnetism; a glassy, very dense, strongly magnetic piece could instead be slag or an iron-rich industrial fragment.
