igneous
Porphyritic Basalt River Stone
Porphyritic Basalt
Hardness: 6-7 (Mohs scale); Color: Dark grey to charcoal matrix with white/light grey phenocrysts; Luster: Dull to sub-vitreous; Structure: Aphanitic matrix with visible crystals; Specific gravity: 2.8-3.0
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to sub-vitreous
Identified More igneous →
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to charcoal matrix with white/light grey phenocrysts; Luster: Dull to sub-vitreous; Structure: Aphanitic matrix with visible crystals; Specific gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The white spots (phenocrysts), likely plagioclase feldspar, grew slowly in the magma chamber before the eruption. This specimen has been rounded and smoothed by water erosion.
Uses & applications
Commonly used as decorative garden stones, in landscaping, or as massage stones. In crushed form, basalt is vital for road construction and concrete aggregate.
Geological facts
Basalt is the most common rock in Earth's crust and makes up most of the ocean floor. Large phenocrysts in a fine-grained 'groundmass' create the porphyritic texture seen here, often called 'snowflake' or 'flower' stone in certain regions.
Field identification & locations
Identify by the high density (heavy for its size), dark color, and the presence of distinct light-colored crystals embedded in a smooth, water-worn surface. Commonly found in riverbeds or coastal areas near volcanic regions.
