igneous
Vesicular Basalt (Scoria)
Mafic extrusive igneous rock (primarily Plagioclase Feldspar and Pyroxene)
Hardness: 5-6 Mohs; Color: Dark grey to reddish-brown (from oxidation); Luster: Dull/Earthy; Structure: Vesicular (pitted with gas bubbles); Cleavage: None; Specific Gravity: 2.5-3.0
- Hardness
- 5-6 Mohs
- Color
- Dark grey to reddish-brown (from oxidation)
- Luster
- Dull/Earthy
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Physical properties
Hardness: 5-6 Mohs; Color: Dark grey to reddish-brown (from oxidation); Luster: Dull/Earthy; Structure: Vesicular (pitted with gas bubbles); Cleavage: None; Specific Gravity: 2.5-3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. The holes (vesicles) are created by gas bubbles that were trapped as the lava solidified. These typically form near volcanic vents or at the surface of lava flows.
Uses & applications
Used in landscaping (lava rock), high-temperature insulation, drainage systems, and as an additive in lightweight concrete. In jewelry, it is sometimes used for 'lava bead' bracelets.
Geological facts
Scoria is similar to pumice, but because it contains more iron and magnesium, it is heavier and sinks in water, whereas pumice often floats. The reddish tint specifically comes from the oxidation of iron within the rock after it was erupted.
Field identification & locations
Identify by its 'sponge-like' appearance and relatively heavy weight compared to its size. Commonly found in volcanic regions such as Hawaii, Iceland, and the American Southwest (Arizona/Utah). Field identification relies on checking for holes and a dark, mafic color.
