igneous
Basalt River Stone with Mineral Crust
Aphanitic Mafic Igneous Rock
Hardness: 6-7 Mohs; Color: Dark grey to black matrix with white/tan surface deposits; Luster: Dull to sub-metallic; Structure: Fine-grained (aphanitic); Shape: Smooth, rounded water-worn cobble.
- Hardness
- 6-7 Mohs
- Color
- Dark grey to black matrix with white/tan surface deposits
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey to black matrix with white/tan surface deposits; Luster: Dull to sub-metallic; Structure: Fine-grained (aphanitic); Shape: Smooth, rounded water-worn cobble.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the earth's surface. The smooth shape indicates long-term transport in a high-energy aquatic environment, such as a river or coastline. The white spotting is likely calcium carbonate or salt crusting from evaporative processes.
Uses & applications
Commonly used in landscaping, decorative masonry, massage therapy stones, and as aggregate in construction and road building.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The white spots on the surface are secondary mineral precipitates, likely lichen or caliche, rather than part of the rock's primary cooling structure.
Field identification & locations
Identify in the field by its heavy weight (high density), dark color, and lack of visible large crystals. Commonly found in volcanic regions and transported via river systems to beaches.
