igneous
Basalt River Pebble
Basalt (Aphanitic Mafic Volcanic Rock)
Hardness: 6-7 Mohs; Color: Dark grey to charcoal (often appears black when wet); Luster: Dull to earthy; Crystal Structure: Microcrystalline/Aphanitic (crystals too small to see with the naked eye); Cleavage: None; Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 Mohs
- Color
- Dark grey to charcoal (often appears black when wet)
- Luster
- Dull to earthy
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey to charcoal (often appears black when wet); Luster: Dull to earthy; Crystal Structure: Microcrystalline/Aphanitic (crystals too small to see with the naked eye); 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. The rounded shape indicates long-term transport and mechanical weathering in a high-energy fluvial (river) or coastal environment.
Uses & applications
Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Smooth river pebbles are highly popular in landscaping, aquarium decor, and zen gardens.
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, where dark 'seas' or maria are actually vast plains of basaltic lava.
Field identification & locations
Identify by its heavy weight (density), dark uniform color, and lack of visible grains. Common in volcanic regions, riverbeds, and glacial till. Collectors look for unique shapes or vesicles (gas bubbles) filled with secondary minerals like quartz or zeolites.
