igneous
Basalt with Olivine
Aphanitic Basalt with (Mg,Fe)2SiO4 phenocrysts
Hardness: 6 (basalt matrix) to 7 (olivine); Color: Dark grey to black matrix with olive-green inclusions; Luster: Dull/earthy matrix with vitreous inclusions; Structure: Aphanitic (fine-grained) with visible phenocrysts; Cleavage: None (conchoidal fracture).
- Hardness
- 6 (basalt matrix) to 7 (olivine)
- Color
- Dark grey to black matrix with olive-green inclusions
- Luster
- Dull/earthy matrix with vitreous inclusions
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Physical properties
Hardness: 6 (basalt matrix) to 7 (olivine); Color: Dark grey to black matrix with olive-green inclusions; Luster: Dull/earthy matrix with vitreous inclusions; Structure: Aphanitic (fine-grained) with visible phenocrysts; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The green olivine crystals crystallized early in the cooling process while the magma was still underground (phenocrysts). Ages vary by location, often associated with Cenozoic volcanic activity.
Uses & applications
Predominantly used as crushed stone for construction, road base, and railroad ballast. Gem-quality olivine (peridot) is used in jewelry, though these small inclusions are for specimen collecting.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Olivine is one of the first minerals to crystallize from a melt and is a primary component of the Earth's upper mantle.
Field identification & locations
Identify by the dark, heavy, fine-grained matrix and the distinctive glassy green 'sand' or crystals embedded within. Commonly found in volcanic fields like those in Hawaii, Arizona, or Iceland. Collectors look for high contrast between the green and black.
