igneous
Basalt (River Pebble)
Extrusive Mafic Igneous Rock
Hardness: 6-7 (Mohs scale); Color: Dark grey to black; Luster: Dull or earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0; Texture: Smooth due to water erosion.
- Hardness
- 6-7 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull or earthy
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to black; Luster: Dull or earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0; Texture: Smooth due to water erosion.
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 polished and rounded by water transport in a river or coastal environment over thousands of years.
Uses & applications
Commonly used as high-quality aggregate in construction, road base, and concrete. In polished pebble form, it is used for massage therapy (hot stones), landscaping, and decorative aquariums.
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.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture. It is frequently found as rounded cobbles in riverbeds or on volcanic beaches. Collectors value unusual shapes or those with high-contrast mineral inclusions.
