igneous
Basalt with inclusion
Basalt (Aphanitic-Porphyritic texture)
Hardness: 5-6 (Mohs); Color: Dark grey, charcoal, or dark brown/purple (weathered); Luster: Dull to earthy; Crystal structure: Fine-grained matrix with larger visible phenocrysts (porphyritic); Specific gravity: 2.8-3.0.
- Hardness
- 5-6 (Mohs)
- Color
- Dark grey, charcoal, or dark brown/purple (weathered)
- Luster
- Dull to earthy
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark grey, charcoal, or dark brown/purple (weathered); Luster: Dull to earthy; Crystal structure: Fine-grained matrix with larger visible phenocrysts (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 a moon. This specimen appears water-worn from a beach environment, likely Cenozoic in age.
Uses & applications
Used primarily in construction for aggregate, road base, and railroad ballast. Exceptionally smooth pieces are used in lapidary work or as river stones for landscaping.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is the primary rock type of the Hawaiian Islands.
Field identification & locations
Identify in the field by its high density, dark color, and fine-grained texture. On beaches, look for smooth, rounded dark stones that feel heavier than they look.
