igneous

Pumice

Pumice (a highly vesicular volcanic glass)

Mohs Hardness: 5 to 6; Color: Light gray to yellowish white; Luster: Dull to pearly; Crystal Structure: Amorphous (glassy); Cleavage: None; Density: Low enough to float on water.

Hardness: 5 to 6
Color: Light gray to yellowish white
Luster: Dull to pearly

Identified May 3, 2026More igneous →

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Physical properties

Mohs Hardness: 5 to 6; Color: Light gray to yellowish white; Luster: Dull to pearly; Crystal Structure: Amorphous (glassy); Cleavage: None; Density: Low enough to float on water.

Formation & geological history

Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected and rapidly cools. The rapid depressurization creates bubbles (vesicles) while the magma solidifies into glass.

Uses & applications

Used as an abrasive in polishing and cleaning, in beauty products for exfoliation, in horticulture as an aerating agent, and as a lightweight aggregate in concrete.

Geological facts

Pumice is the only rock that can float on water until it becomes waterlogged. After the Krakatoa eruption in 1883, rafts of pumice drifted across the ocean for up to 20 years.

Field identification & locations

Identify in the field by its sponge-like appearance and by placing a dry piece in water to see if it floats. Commonly found in areas with past or present explosive volcanic activity.

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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