igneous
Basalt (Weathered Pebble)
Extrusive mafic igneous rock; minerals primarily Plagioclase feldspar and Pyroxene
Hardness: 6 (Mohs scale); Color: Dark grey to greenish-grey (weathered); Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to greenish-grey (weathered)
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to greenish-grey (weathered); 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 specific specimen appears to be a water-worn or weathered pebble likely from a riverbed or coastal environment.
Uses & applications
Primary use is in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Fine specimens are sometimes used in lapidary work as 'river stones' for landscaping or grounding tools.
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 extensively on the Moon and Mars.
Field identification & locations
Identify by its dark color, heavy weight for its size (density), and fine-grained texture. It is commonly found in volcanic regions, oceanic islands (like Hawaii), and riverbeds downstream from volcanic plateaus.
