igneous

Pumice

Vesicular Rhyolite or Dacite

Hardness: 5-6 (Mohs scale), but often crumbles; Color: White, cream, gray, or light brown; Luster: Dull to pearly; Crystal Structure: Amorphous/Glassy; Cleavage: None; Specific Gravity: < 1.0 (floats on water); Texture: Highly vesicular (honeycombed with gas bubbles).

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

Identified March 7, 2026More igneous →

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Physical properties

Formation & geological history

Formed during explosive volcanic eruptions where highly pressurized, gas-rich molten rock (magma) is violently ejected. The rapid cooling and depressurization trap gas bubbles inside the cooling glass. Geological age can range from very recent to hundreds of millions of years in preserved volcanic arcs.

Uses & applications

Used as an abrasive in industrial cleaners, toothpastes, and 'stone-washed' jeans; as a lightweight aggregate in concrete/cinder blocks; in horticulture (soil aeration); and as a cosmetic exfoliant (pumice stones).

Geological facts

Pumice is the only rock that can float on water due to its extreme porosity. Following the 1883 eruption of Krakatoa, rafts of pumice drifted across the Indian Ocean for up to 20 years, some large enough to support growing trees.

Field identification & locations

Identify in the field by its very light weight relative to size and its scratchy, 'foamy' surface texture. Commonly found in volcanic regions like the Cascades (USA), Italy, and New Zealand. Collectors look for light-colored, highly porous specimens without heavy weathering.

Other igneous specimens

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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