igneous
Basalt (Weathered Pebble)
Extrusive mafic rock (primarily Plagioclase Feldspar and Pyroxene)
Hardness: 5-6 (Mohs scale); Color: Dark grey to charcoal with micro-crystalline texture; Luster: Dull/Earthy (weathered); Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0.
- Hardness
- 5-6 (Mohs scale)
- Color
- Dark grey to charcoal with micro-crystalline texture
- Luster
- Dull/Earthy (weathered)
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark grey to charcoal with micro-crystalline texture; Luster: Dull/Earthy (weathered); Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the Earth's surface. These specific specimens are water-worn 'river rocks' or beach pebbles, likely from the Cenozoic era or older volcanic flows.
Uses & applications
Used extensively in construction as road base, aggregate in concrete, and railroad ballast. Exceptionally durable for landscaping and stone carvings.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also a major component of the lunar maria on the Moon and the surface of Mars.
Field identification & locations
Identify in the field by its dark color, heavy weight for its size, and the absence of visible large crystals. Commonly found in volcanic regions, coastal areas, and ancient lava beds. Collectors look for 'vesicular' basalt which contains gas bubbles.
