igneous
Basalt with Quartz/Epidote Vein
Mafic volcanic rock (formula primarily (Mg, Fe)2SiO4 and CaAl2Si2O8)
Hardness: 6-7 Mohs; Color: Black to dark charcoal with a tan/greenish vein; Luster: Dull to sub-vitreous; Structure: Fine-grained (aphanitic) matrix; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8–3.0.
- Hardness
- 6-7 Mohs
- Color
- Black to dark charcoal with a tan/greenish vein
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Black to dark charcoal with a tan/greenish vein; Luster: Dull to sub-vitreous; Structure: Fine-grained (aphanitic) matrix; Cleavage: None (conchoidal fracture); 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 visible vein was formed later when mineral-rich hydrothermal fluids filled cracks in the cooling basalt. Common in volcanic islands and oceanic crust.
Uses & applications
Basalt is widely used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. This specific tumbled specimen is primarily of interest to hobbyist beachcombers or as decorative 'river rock'.
Geological facts
Basalt is the most common rock type in the Earth's crust and also covers large areas of the Moon (the dark 'Maria' patches) and Mars. This specimen has been naturally tumbled and polished by wave action on a beach.
Field identification & locations
Identify in the field by its heavy weight (density), dark color, and lack of visible large crystals. The lighter band 'vein' is a distinct identifying feature of secondary mineralization. Commonly found on volcanic coastlines in the Pacific Northwest, Hawaii, and Iceland.
