Igneous
Basalt (River Cobble)
Mafic Igneous Basalt (composed primarily of Plagioclase feldspar, Pyroxene, and Olivine)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black (weathered to dull brown); Luster: Sub-metallic to dull; Structure: Fine-grained (aphanitic) or porphyritic; Specific Gravity: 2.8–3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black (weathered to dull brown)
- Luster
- Sub-metallic to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black (weathered to dull brown); Luster: Sub-metallic to dull; Structure: Fine-grained (aphanitic) or porphyritic; 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 specimen has been further shaped by fluvial (river) erosion, which resulted in its smooth, rounded cobble shape over thousands of years.
Uses & applications
Commonly used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Smooth cobbles are often used in landscaping and garden design.
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 common on other planetary bodies like the Moon, Mars, and Venus.
Field identification & locations
Identify in the field by its heavy weight (high density), dark interior color, and lack of visible large crystals. Commonly found in volcanic regions or in riverbeds downstream from volcanic plateaus. Collectors look for vesicles (air bubbles) or phenocrysts (embedded crystals) inside.
