igneous
Basalt (Polished pebble)
Basalt (Aphanitic mafic volcanic rock)
Hardness: 6.0 on Mohs scale; Color: Dark grey to black with light plagioclase phenocrysts; Luster: Dull to waxy when tumbled; Structure: Fine-grained (aphanitic) matrix; Specific Gravity: 2.8–3.0.
- Hardness
- 6
- Color
- Dark grey to black with light plagioclase phenocrysts
- Luster
- Dull to waxy when tumbled
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Physical properties
Hardness: 6.0 on Mohs scale; Color: Dark grey to black with light plagioclase phenocrysts; Luster: Dull to waxy when tumbled; Structure: Fine-grained (aphanitic) matrix; Specific Gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Examples like this pebble are often smoothed by fluvial or marine erosion over thousands of years.
Uses & applications
Used extensively in construction as road base, railroad ballast, and concrete aggregate. Polished stones like this are used for hot stone massage, decorative landscaping, and beginner rock collections.
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 found in abundance on the Moon and Mars.
Field identification & locations
Identify in the field by its high density, dark color, and lack of visible large quartz crystals. Commonly found on volcanic islands, ocean beaches, and riverbeds in volcanic regions.
