igneous
Basalt with Olivine Phenocrysts
Porphyritic Basalt (containing Mg2SiO4 - Fe2SiO4)
Hardness: 6-7 (Mohs scale); Color: Dark grey to black matrix with green crystalline inclusions; Luster: Dull matrix with vitreous inclusions; Crystal structure: Orthorhombic (olivine); Specific gravity: 2.8–3.4.
- Hardness
- 6-7 (Mohs scale)
- Color
- Dark grey to black matrix with green crystalline inclusions
- Luster
- Dull matrix with vitreous inclusions
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to black matrix with green crystalline inclusions; Luster: Dull matrix with vitreous inclusions; Crystal structure: Orthorhombic (olivine); Specific gravity: 2.8–3.4.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. This specific specimen’s rounded shape suggests it was eroded and transported by water (river or beach pebble). Age is variable depending on the volcanic field.
Uses & applications
Basalt is primarily used in construction as crushed stone for road bases and concrete aggregate. Olivine-rich basalt is sometimes used in sandblasting or as a gemstone (Peridot) if crystals are large and clear.
Geological facts
When olivine is found in volcanic rocks, it is often one of the first minerals to crystallize out of the magma. Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor.
Field identification & locations
Identify in the field by looking for small, glassy green 'grains' (olivine) embedded in a dense, dark, heavy rock. Common in volcanic regions such as Hawaii, Iceland, and the Pacific Northwest. Collectors should look for higher-density ‘green sand’ beaches nearby.
