igneous
Pumice
Pumicite
Hardness: 5-6 (but very brittle); Color: Grey, white, or light tan; Luster: Dull to pearly; Crystal Structure: Amorphous (volcanic glass); Highly vesicular (full of holes); Specific Gravity: 0.25 to 0.7 (floats on water).
- Hardness
- 5-6 (but very brittle)
- Color
- Grey, white, or light tan
- Luster
- Dull to pearly
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Physical properties
Hardness: 5-6 (but very brittle); Color: Grey, white, or light tan; Luster: Dull to pearly; Crystal Structure: Amorphous (volcanic glass); Highly vesicular (full of holes); Specific Gravity: 0.25 to 0.7 (floats on water).
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected rapidly. The sudden depressurization causes the gas to expand into bubbles as the lava cools almost instantly, trapping a frothy texture. It is found in volcanic regions of all ages, from modern to millions of years old.
Uses & applications
Used widely as an abrasive in polishing, pencil erasers, and skincare (exfoliation). In construction, it is used as a lightweight aggregate for concrete and cinder blocks. It is also common in horticulture as a soil amendment.
Geological facts
Pumice is the only rock that can float on water until it eventually becomes waterlogged and sinks. During large volcanic eruptions, vast 'pumice rafts' can form on the ocean, stretching for miles and visible from space.
Field identification & locations
Identify in the field by its sponge-like appearance and extremely light weight. If it floats in a puddle or stream, it is likely pumice. Commonly found in areas with past rhyolitic or andesitic volcanic activity.
