igneous
Riverstone (Gray Basalt)
Basalt (Maier-type or generic)
Hardness: 5-6 (Mohs); Color: Dark gray to black; Luster: Dull or earthy when unpolished; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0.
- Hardness
- 5-6 (Mohs)
- Color
- Dark gray to black
- Luster
- Dull or earthy when unpolished
Identified More igneous →
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark gray to black; Luster: Dull or earthy when unpolished; 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 has been smooth-rounded by fluvial erosion (water transport) in a river or stream over hundreds to thousands of years.
Uses & applications
Common riverstones are used in landscaping, construction aggregate, aquarium decor, and sometimes as 'worry stones' or for hot stone massage when polished.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is similar in composition to rocks found on the Moon and Mars.
Field identification & locations
Identify by its heavy weight relative to size, fine-grained texture, and dark color. Common in volcanic regions and riverbeds. Collectors look for unique shapes or basalt containing olivine crystals.
