igneous

Pumice

Pumice (a highly vesicular volcanic glass)

Hardness: 5-6 (Mohs), but brittle; Color: White, cream, or light gray; Luster: Dull to pearly; Crystal structure: Amorphous (glassy); Cleavage: None; Specific Gravity: less than 1.0 (it floats on water)

Hardness: 5-6 (Mohs), but brittle
Color: White, cream, or light gray
Luster: Dull to pearly

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected into the air; the rapid cooling and depressurization trap gas bubbles in a frothy glass structure. It is found in volcanic regions of any geological age from Holocene to ancient.

Uses & applications

Used as a lightweight aggregate in concrete, as an abrasive in industrial polishing and 'stone-washed' jeans, in horticultural soil mixes to improve aeration, and as a cosmetic exfoliant (pumice stones).

Geological facts

Pumice is the only rock that can float on water until it eventually becomes waterlogged and sinks. Large rafts of pumice have been known to float in the ocean for years following major volcanic eruptions like Krakatoa.

Field identification & locations

Identify in the field by its extreme lightness (low density) and frothy, sponge-like texture. It is commonly found in volcanic fields such as those in the American West, Italy, Iceland, and New Zealand.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous