igneous
Basalt Pebble (Vesicular / Beach Cobble)
Basalt (mafic extrusive igneous rock)
Hardness: 6 (Mohs scale); Color: Dark greenish-grey to black; Luster: Dull to sub-vitreous when wet; Structure: Aphanitic (fine-grained) with small pitted vesicles; Specific gravity: 2.8-3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark greenish-grey to black
- Luster
- Dull to sub-vitreous when wet
Identified More igneous →
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark greenish-grey to black; Luster: Dull to sub-vitreous when wet; Structure: Aphanitic (fine-grained) with small pitted vesicles; Specific gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of magnesium- and iron-rich lava exposed at or near the surface of Earth. The rounded form results from coastal wave erosion tumbling the rock against sand and gravel.
Uses & applications
Used in construction as crushed aggregate, road base, asphalt concrete, and as a popular decorative rock for landscaping or beach rock collections.
Geological facts
Basalt is the most common rock type in the Earth's crust, forming the majority of the ocean floor. Small cavities called vesicles are left behind by gas bubbles trapped in the lava as it cooled.
Field identification & locations
Identified by its heavy weight, fine grain, dark grey-green tone, and tiny surface pits from gas bubbles. Commonly found on volcanic ocean beaches, riverbeds, and glacial deposits worldwide.
