igneous
Basalt with Quartz Vein and Weathering Crust
Amygdaloidal or Porphyritic Basalt
Hardness: 5-6 (Mohs); Color: Dark grey to black interior, tan/buff weathered exterior; Luster: Dull to sub-vitreous; Structure: Fine-grained aphanitic with a distinct mineral vein.
- Hardness
- 5-6 (Mohs)
- Color
- Dark grey to black interior, tan/buff weathered exterior
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark grey to black interior, tan/buff weathered exterior; Luster: Dull to sub-vitreous; Structure: Fine-grained aphanitic with a distinct mineral vein.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The internal vein likely formed later via hydrothermal activity where mineral-rich water filled cracks in the cooling rock. This specimen shows significant river-worn rounding.
Uses & applications
Primarily used in construction (crushed stone, road base) and as a tool stone historically. This specific specimen has decorative/collecting value as a curiosity.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The tan layer on the left side is a 'weathering rind' caused by chemical weathering over thousands of years.
Field identification & locations
Identify by its heavy weight (high density), dark fine-grained interior, and the presence of a distinct 'rind' or mineral bands. Commonly found in volcanic regions, riverbeds, and glacial tills.
