Vesicular Basalt (Scoria)

Physical Properties

Hardness: 5-6 (Mohs); Color: Dark grey to black; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained) with large vesicules (gas bubbles); Cleavage: None/Irregular

Formation & Geological History

Formed from the rapid cooling of basaltic lava rich in dissolved gases. As the lava depressurizes at the surface, gases expand to form bubbles which are trapped as the rock solidifies. Common in Cenozoic volcanic fields.

Uses & Applications

Used in landscaping properly referred to as 'lava rock', high-temperature insulation, drainage systems, and as an abrasive in 'stone-washed' denim production.

Geological Facts

The holes (vesicles) occur because lava is under high pressure underground; when it erupts, it is like opening a carbonated soda bottle, causing gas to fizz out and create voids.

Field Identification & Locations

Identify in the field by its dark color, heavy weight relative to pumice (it will sink in water), and the presence of numerous circular or oval cavities. Found near volcanic hotspots and rift zones globally.

Identified on: 3/21/2026

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