igneous
Vesicular Basalt (Scoria)
Basalt (Extrusive Igneous Rock)
Hardness: 5-6 Mohs. Color: Black, dark gray, or reddish-brown. Luster: Dull to sub-metallic. Crystal structure: Aphanitic (fine-grained) with a vesicular (pitted) texture. Specific gravity: 2.8-3.0.
- Hardness
- 5-6 Mohs
- Color
- Black, dark gray, or reddish-brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 Mohs. Color: Black, dark gray, or reddish-brown. Luster: Dull to sub-metallic. Crystal structure: Aphanitic (fine-grained) with a vesicular (pitted) texture. Specific gravity: 2.8-3.0.
Formation & geological history
Formed during explosive volcanic eruptions or from the cooling of gas-rich lava flows. As the lava cools rapidly on the surface, trapped gas bubbles (vesicles) form the characteristic holes. It can be found in volcanic regions of any geological age, from modern flows to ancient Precambrian belts.
Uses & applications
Used in landscaping as decorative lava rock, as an aggregate in gas grills to retain heat, in concrete lightweight aggregate, and occasionally in road construction. High-quality specimens are kept in mineral collections.
Geological facts
The holes in vesicular basalt can sometimes be filled with secondary minerals like zeolites, quartz, or calcite, turning the rock into 'amygdaloidal basalt.' This specific specimen appears to have significant internal mineralization or a contact zone with lighter rhyolitic material.
Field identification & locations
Identified by its dark color, heavy weight (relative to pumice), and presence of frequent gas bubbles or vesicles. Commonly found in volcanic fields like those in Iceland, Hawaii, or the Cascade Range. Field collectors should look for basaltic flows near cinder cones.
