igneous
Basalt (Arrowhead form)
Extrusive mafic igneous rock
Hardness: 6 (Mohs); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0.
- Hardness
- 6 (Mohs)
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs); Color: Dark grey to black; Luster: Dull to sub-metallic; 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 surface of a terrestrial planet or the moon. Most basalt on Earth is from the Cenozoic era, though it spans all geological ages.
Uses & applications
Historically used for stone tools (lithics) like the pictured arrowhead. Modernly used in construction as aggregate, floor tiles, and in the production of stone wool insulation.
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 large quantities on the Moon and Mars.
Field identification & locations
Identify by its dark color, heavy weight (high density), and fine-grained texture. In the field, it often exhibits columnar jointing. This specific specimen shows evidence of conchoidal fracturing and knapping by humans to create a point.
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