igneous
Basalt
Basalt (primarily composed of Plagioclase Feldspar and Pyroxene)
Hardness: 6-7 (Mohs); Color: Dark grey to black, weathering to brown; Luster: Dull or earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8 - 3.0
- Hardness
- 6-7 (Mohs)
- Color
- Dark grey to black, weathering to brown
- Luster
- Dull or earthy
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark grey to black, weathering to brown; Luster: Dull or earthy; Crystal structure: Aphanitic (fine-grained); 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 Earth's surface. Common in oceanic crust and volcanic regions; ages range from modern to billions of years old.
Uses & applications
Extensively used in construction as an aggregate (crushed stone) for roads, railroad ballast, and concrete. Also used in high-quality floor tiles and monuments.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon and Mars.
Field identification & locations
Identify by its dark color, heavy weight (density), and fine-grained texture with no visible crystals to the naked eye. Commonly found in coastal areas or old volcanic plateaus. Collectors look for vesicles (holes) which may contain secondary minerals like zeolites or semi-precious gems.
