igneous
Basalt (Greenstone)
Extrusive mafic igneous rock
Hardness: 6 (Mohs scale); Color: Dark gray to greenish-gray; Luster: Dull/earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray to greenish-gray
- Luster
- Dull/earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark gray to greenish-gray; Luster: Dull/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. This specimen shows a greenish hue, suggesting low-grade metamorphism into greenstone over millions of years.
Uses & applications
Commonly used as aggregate in highway construction, railroad ballast, and in the manufacturing of stone wool insulation. Also used for high-durability floor tiles.
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 in abundance on the Moon and Mars.
Field identification & locations
Identified by its heavy weight (high density), fine grain, and dark color. It is found globally, particularly in volcanic regions like the Pacific Northwest, Iceland, and Hawaii. Collectors look for vesicles or mineral inclusions.
