Igneous
Basalt with Olivine Phenocrysts
Porphyritic Basalt (Mineral formula of Olivine: (Mg,Fe)2SiO4)
Hardness: 6 (basalt) to 7 (olivine inclusions) on the Mohs scale. Color: Dark gray to black matrix with olive-green inclusions. Luster: Dull or earthy (matrix), vitreous (crystals). Crystal structure: Orthorhombic (olivine), aphanitic (matrix). Cleavage: Poor. Specific Gravity: 2.8-3.0.
- Hardness
- 6 (basalt) to 7 (olivine inclusions) on the Mohs scale
- Color
- Dark gray to black matrix with olive-green inclusions
- Luster
- Dull or earthy (matrix), vitreous (crystals)
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Physical properties
Hardness: 6 (basalt) to 7 (olivine inclusions) on the Mohs scale. Color: Dark gray to black matrix with olive-green inclusions. Luster: Dull or earthy (matrix), vitreous (crystals). Crystal structure: Orthorhombic (olivine), aphanitic (matrix). Cleavage: Poor. 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. The olive-green crystals (phenocrysts) formed deep underground before the volcanic eruption brought them to the surface. Most basalt is Cenozoic to Holocene in age.
Uses & applications
Industrial use as crushed stone for road construction, concrete aggregate, and railroad ballast. Exceptionally clear olivine crystals are cut into 'peridot' gemstones. Porphyritic specimens are popular for educational purposes.
Geological facts
Basalt is the most common rock on Earth's surface and makes up most of the ocean floor. It is also extremely common on the Moon and Mars. When the green olivine crystals are large, they are sometimes called 'lava gems'.
Field identification & locations
Identify by looking for a fine-grained, dark, heavy matrix contain tiny glassy green grains. Common in volcanic regions such as Hawaii, Iceland, and the Pacific Northwest (Columbia River Basalt Group). Collectors should look for high contrast between the dark matrix and the green minerals.
