igneous
Conchoidal Basalt Pebble
Basalt with conchoidal fracturing
Hardness: 6-7 on Mohs scale; Color: Dark grey to charcoal black; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained); Cleavage: None, exhibits curved conchoidal fractures; Specific Gravity: 2.8–3.0
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to charcoal black
- Luster
- Dull to sub-vitreous
Identified More igneous →
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to charcoal black; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained); Cleavage: None, exhibits curved conchoidal fractures; 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 specimen shown is a water-worn river or beach pebble that has been smoothed by mechanical erosion over time.
Uses & applications
Primarily used in construction (crushed stone), road base, and as decorative river rock. Fine-grained basalt is also used for hot stone massage therapy and high-end landscaping.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The unique curved patterns on this specific stone are likely 'conchoidal' fractures, the same type of breaking pattern found in flint or obsidian.
Field identification & locations
Identify by its dark color, heavy weight for its size, and absence of visible crystals. It is commonly found on volcanic beaches and in glacial till throughout the Pacific Northwest and Great Lakes regions.
