igneous
Scoria (Lava Rock)
Mafic extrusive volcanic rock (Scientific formula varies based on mineral content: primarily SiO2, MgO, and FeO)
Hardness: 5-6 Mohs. Color: Dark brown, black, or reddish-purple. Luster: Dull/Earthy. Structure: Highly vesicular (contain gas bubbles). Cleavage: None. Specific gravity: Low (often sinks in water, unlike pumice).
- Hardness
- 5-6 Mohs
- Color
- Dark brown, black, or reddish-purple
- Luster
- Dull/Earthy
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Physical properties
Hardness: 5-6 Mohs. Color: Dark brown, black, or reddish-purple. Luster: Dull/Earthy. Structure: Highly vesicular (contain gas bubbles). Cleavage: None. Specific gravity: Low (often sinks in water, unlike pumice).
Formation & geological history
Formed from the rapid cooling of basaltic or andesitic lava rich in dissolved gas. As the lava is ejected from a volcano, the pressure drops and gas expands, creating holes (vesicles) that are frozen in place as the rock solidifies.
Uses & applications
Commonly used in landscaping (lava rocks), high-temperature insulation, drainage systems, gas grills, and as a lightweight aggregate in concrete blocks.
Geological facts
Scoria is often mistaken for pumice, but it is much denser and darker because it contains more iron and magnesium. It is the primary component of 'cinder cones,' which are steep, conical volcanic hills.
Field identification & locations
Identify by its 'Swiss cheese' appearance, dark color, and rough, abrasive texture. It is common in active volcanic regions like Hawaii, Iceland, and the American Southwest. Collectors should look for pieces with distinct, large vesicles.
