igneous
Basalt with Quartz/Calcite Veins
Aphanitic Mafic Igneous Rock with Hydrothermal Veining
Hardness: 5.5-6 (basalt matrix), 7 (quartz) or 3 (calcite); Color: Dark grey to black matrix with white or cream veins; Luster: Dull to sub-vitreous; Structure: Fine-grained aphanitic matrix; Specific Gravity: 2.8-3.0
- Hardness
- 5
- Color
- Dark grey to black matrix with white or cream veins
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 5.5-6 (basalt matrix), 7 (quartz) or 3 (calcite); Color: Dark grey to black matrix with white or cream veins; Luster: Dull to sub-vitreous; Structure: Fine-grained aphanitic matrix; Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of basaltic lava. The white veins are secondary hydrothermal deposits where mineral-rich water filled cracks and crystallized during or after solidifying. Commonly found in volcanic terrains and coastal riverbeds.
Uses & applications
Primarily used in construction (crushed stone, road base) and occasionally as decorative river stones or pocket stones for collectors.
Geological facts
The 'veins' in the rock act like a geological time capsule, representing a period of tectonic stress that cracked the stone, followed by mineral precipitation from over-pressurized fluids.
Field identification & locations
Identify in the field by the high contrast between the dark, fine-grained host rock and the white crystalline streaks. Testing with weak acid can determine if veins are calcite (fizzes) or quartz (no reaction).
