Igneous
Basalt
Mafic volcanic rock, primarily composed of Plagioclase, Pyroxene, and Olivine
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic (glassy when quenched); Crystal Structure: Fine-grained (aphanitic) or porphyritic; Specific Gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic (glassy when quenched)
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic (glassy when quenched); Crystal Structure: Fine-grained (aphanitic) or porphyritic; Specific Gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the surface of a terrestrial planet or moon. Most basalt flows are from the Cenozoic era, though it has formed throughout Earth's history.
Uses & applications
Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. It is also used in high-quality floor tiles and as 'hot stones' in massage therapy.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up the majority of the ocean floor. It is also found on the Moon and Mars, forming the dark 'maria' plains visible from Earth.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and lack of visible crystals to the naked eye. Commonly found in volcanic regions like Hawaii, Iceland, or the Deccan Traps. Collectors look for vesicles (gas bubbles) that may contain secondary minerals like zeolites or amethysts.
