igneous
Basalt (Porphyritic)
Porphyritic Basalt
Hardness: 6 (Mohs scale); Color: Dark grey to black with light grey/white crystals; Luster: Dull to sub-metallic; Crystal structure: Aphanitic matrix with phenocrysts; Cleavage: None; Specific gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black with light grey/white crystals
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black with light grey/white crystals; Luster: Dull to sub-metallic; Crystal structure: Aphanitic matrix with phenocrysts; Cleavage: None; Specific gravity: 2.8 - 3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. The larger crystals (phenocrysts) formed at depth before the lava erupted and the rest cooled quickly as a fine-grained matrix.
Uses & applications
Primarily used as aggregate in construction, road base, and concrete. This specific water-worn pebble type is often used for massage stones or decorative landscaping.
Geological facts
Basalt is the most common rock in Earth's crust and makes up most of the ocean floor. It is also found extensively on the Moon and Mars.
Field identification & locations
Identify by its dark, heavy nature and small visible crystals (phenocrysts). Commonly found on volcanic beaches or in riverbeds draining volcanic mountain ranges. Collectors look for unique phenocryst patterns.
