igneous
Basalt Porphyry (River Pebble)
Porphyritic Basalt
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal with light-colored phenocrysts; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained) groundmass with visible macro-crystals (plagioclase feldspar); Specific gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to charcoal with light-colored phenocrysts
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal with light-colored phenocrysts; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained) groundmass with visible macro-crystals (plagioclase feldspar); Specific gravity: 2.8–3.0.
Formation & geological history
Formed through rapid cooling of mafic lava on the Earth's surface. The larger white spots (phenocrysts) formed earlier at depth through slower cooling before the final eruption. This specific specimen has been rounded and smoothed by water erosion in a river or coastal environment.
Uses & applications
Used in construction as crushed stone or aggregate. Historically used for paving stones. Smooth pebbles are popular in landscaping, aquariums, and rock tumbling.
Geological facts
Basalt is the most common rock in the Earth's crust, making up most of the ocean floor and large volcanic plateaus. Porphyritic textures tell a 'two-stage' cooling story of the volcano's history.
Field identification & locations
Identify in the field by its dark, dense matrix and the presence of lighter, blocky crystals. Commonly found in volcanic regions and transported by rivers. Collectors look for high-contrast 'snowflake' patterns.
