igneous
Basalt Pebble
Basalt (Aphanitic Mafic Igneous Rock)
Hardness: 6 on Mohs scale; Color: Dark grey to black (weathered to light grey); Luster: Dull/Earthy; Crystal Structure: Very fine-grained (aphanitic) microcrystalline; Cleavage: None; Specific Gravity: 2.8–3.0
- Hardness
- 6 on Mohs scale
- Color
- Dark grey to black (weathered to light grey)
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6 on Mohs scale; Color: Dark grey to black (weathered to light grey); Luster: Dull/Earthy; Crystal Structure: Very fine-grained (aphanitic) microcrystalline; Cleavage: None; 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 surface of a terrestrial planet or a moon. This specimen shows river or beach erosion (rounding) indicating it was transported by water long after its initial volcanic formation.
Uses & applications
Used extensively in construction as aggregate (crushed stone) for roads, railroad ballast, and concrete. In polished form, it is used for decorative landscaping and massage stones.
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 on the Moon, Mars, and Venus.
Field identification & locations
Identify in the field by its dark interior, weight (denser than granite), and lack of visible large crystals. Commonly found in volcanic fields, riverbeds draining volcanic areas, and coastal regions with basaltic cliffs.
