igneous
Basalt (Vesicular/Amygdaloidal)
Basalt
Hardness: 5-6 (Mohs); Color: Dark grey to black with light circular spots (amygdules); Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained matrix); Specific Gravity: 2.8-3.0.
- Hardness
- 5-6 (Mohs)
- Color
- Dark grey to black with light circular spots (amygdules)
- Luster
- Dull/Earthy
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark grey to black with light circular spots (amygdules); Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained matrix); Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the surface. The light spots (amygdules) are formed when secondary minerals like quartz or calcite fill gas bubbles (vesicles) in the cooling lava.
Uses & applications
Commonly used as crushed stone for road base, concrete aggregate, and railroad ballast. Exceptionally smooth specimens are used in decorative landscaping or as massage stones.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is juga found on the Moon and Mars.
Field identification & locations
Identify by its heavy weight, dark color, and find it in volcanic regions or along riverbeds where erosion has smoothed the rock into river pebbles.
