igneous
River Cobble (likely Basalt or Greenstone)
Mafic Igneous Rock (with possible chloritic alteration)
Hardness: 5-7 (Mohs). Color: Dark olive green to greyish-green. Luster: Dull to waxy when wet. Structure: Fine-grained (aphanitic), massive. Cleavage: None. Specific Gravity: 2.7-3.3.
- Hardness
- 5-7 (Mohs)
- Color
- Dark olive green to greyish-green
- Luster
- Dull to waxy when wet
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Physical properties
Hardness: 5-7 (Mohs). Color: Dark olive green to greyish-green. Luster: Dull to waxy when wet. Structure: Fine-grained (aphanitic), massive. Cleavage: None. Specific Gravity: 2.7-3.3.
Formation & geological history
Formed from the rapid cooling of magnesium and iron-rich lava. This specimen shows significant river-worn rounding (attrition) and potentially 'greenstone' alteration where minerals like chlorite and epidote have replaced original minerals over millions of years.
Uses & applications
Used primarily as construction aggregate, road base, and in river landscaping. Natural weathered cobbles are popular for decorative gardens and aquarium decor.
Geological facts
Rocks that appear olive green in riverbeds are often ancient basaltic lavas that have undergone low-grade metamorphism. The smooth surface is the result of mechanical weathering in high-energy fluvial environments.
Field identification & locations
Identify by its heavy weight relative to size (high density) and dark interior color. Look for it along fast-moving river banks or glacial till regions. Collectors value smooth, perfectly ellipsoidal shapes.
