igneous
Basalt Pebble
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black, often appearing darker when wet; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic (fine-grained); Cleavage: None; Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black, often appearing darker when wet
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black, often appearing darker when wet; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic (fine-grained); Cleavage: None; 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 specific specimen has been basalt water-worn into a smooth pebble by fluvial or coastal erosion over hundreds to thousands of years.
Uses & applications
Predominantly used as crushed stone for construction, road base, and railroad ballast. Smooth river-worn specimens are popular for landscaping, decorative gardens, and hot stone massage therapy.
Geological facts
Basalt is the most common rock type on Earth's crust and makes up most of the ocean floor. It is also found extensively on the Moon (the dark 'Maria') and Mars.
Field identification & locations
Identify in the field by its dark color, heavy weight (dense), and lack of visible crystals to the naked eye. Commonly found on volcanic beaches, riverbeds near volcanic ranges, and oceanic islands like Iceland or Hawaii.
