igneous
Pumice (Basaltic/Scoria)
Vesicular Basalt / Scoria
Hardness: 5-6 Mohs. Color: Dark grey to black. Luster: Dull/earthy. Crystal structure: Aphanitic/Vesicular (fine-grained with gas bubbles). Specific Gravity: 2.5-3.0 (heavier than light pumice due to iron).
- Hardness
- 5-6 Mohs
- Color
- Dark grey to black
- Luster
- Dull/earthy
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Physical properties
Hardness: 5-6 Mohs. Color: Dark grey to black. Luster: Dull/earthy. Crystal structure: Aphanitic/Vesicular (fine-grained with gas bubbles). Specific Gravity: 2.5-3.0 (heavier than light pumice due to iron).
Formation & geological history
Formed from the rapid cooling of gas-rich mafic lava. As pressure drops during a volcanic eruption, gases dissolve out of the melt, creating bubbles (vesicles) that are frozen in place as the rock solidifies.
Uses & applications
Used in landscaping, high-temperature insulation, drainage in gardening, and as an abrasive in industrial grit and stone-washing textiles.
Geological facts
Unlike white rhyolitic pumice, dark scoria often sinks in water because its vesicle walls are thicker and denser. It is a primary component of 'lava rocks' used in gas grills.
Field identification & locations
Identify by its 'Swiss cheese' appearance and dark color. Common in volcanic regions like Hawaii, Iceland, and the Pacific Northwest. Collectors look for unique aerodynamic 'bomb' shapes.
