igneous
Vesicular Basalt / Scoria (Stream-rounded Cobble)
Mafic extrusive igneous rock
Hardness: 5-6 Mohs. Color: Weathered brown, reddish-brown, dark greyish. Luster: Earthy to dull. Texture: Vesicular (pitted surface with small gas cavities/cavities filled or eroded), fine-grained matrix. Cleavage: None. Specific gravity: 2.3-2.7
- Hardness
- 5-6 Mohs
- Color
- Weathered brown, reddish-brown, dark greyish
- Luster
- Earthy to dull
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Physical properties
Hardness: 5-6 Mohs. Color: Weathered brown, reddish-brown, dark greyish. Luster: Earthy to dull. Texture: Vesicular (pitted surface with small gas cavities/cavities filled or eroded), fine-grained matrix. Cleavage: None. Specific gravity: 2.3-2.7
Formation & geological history
Formed during explosive or effusive volcanic eruptions when gas bubbles (vesicles) were trapped in cooling mafic, basaltic lava. Subsequent stream or river transport rounded the stone into a smooth cobble over time.
Uses & applications
Commonly used as river rock landscaping material, crushed aggregate for construction, and decorative stone. Uncut specimens are often kept by amateur mineral collectors.
Geological facts
The pitted, pocketed surface formed from expanding steam and volcanic gas bubbles trapped as the liquid lava solidified thousands or millions of years ago. Water erosion smoothed its outer form while leaving the pitted interior texture exposed.
Field identification & locations
Identified by its rounded cobble shape combined with distinct, tiny pits or pores (vesicles) and a dull, earthy brownish-grey surface. Commonly found along riverbeds, gravel bars, and ancient volcanic flow plains.
