igneous
Basalt (Phaneritic or Diabase)
Mafic Igneous Rock (composed primarily of Pyroxene and Plagioclase Feldspar)
Hardness: 6-7 Mohs; Color: Dark gray to black with speckled white/gray grains; Luster: Dull to sub-metallic; Structure: Fine to medium-grained (aphanitic to phaneritic); Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 Mohs
- Color
- Dark gray to black with speckled white/gray grains
- Luster
- Dull to sub-metallic
Identified More igneous →
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Physical properties
Hardness: 6-7 Mohs; Color: Dark gray to black with speckled white/gray grains; Luster: Dull to sub-metallic; Structure: Fine to medium-grained (aphanitic to phaneritic); Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. This specimen shows a medium-grained texture typical of a shallow intrusive formation like a dike or sill, often dating from various volcanic epochs throughout Earth's history.
Uses & applications
Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Fine-grained variants are used for floor tiles and monuments.
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 and Mars.
Field identification & locations
Identify by its dark color, heavy weight (density), and lack of visible quartz. In the field, look for hexagonal columnar jointing in large outcrops. Common in volcanic regions like the Pacific Northwest, Hawaii, and Iceland.
