igneous
Porphyritic Basalt or Diabase (Pebble)
Extrusive / Subvolcanic Igneous Rock
Hardness: 6-7 on Mohs scale; Color: Grayish-green matrix with white/light plagioclase feldspar phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic with porphyritic texture; Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grayish-green matrix with white/light plagioclase feldspar phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic with porphyritic texture; Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from cooling of mafic lava flows or shallow intrusions. The rounded shape is due to fluvial or coastal weathering and water transport as a pebble.
Uses & applications
Used in construction, road aggregate, and as a decorative stone.
Geological facts
Porphyritic textures indicate a two-stage cooling history: slow cooling deep underground formed the larger crystals (phenocrysts), followed by rapid eruption and cooling at the surface which formed the fine-grained groundmass.
Field identification & locations
Commonly found in riverbeds and beaches worldwide. Easily identified by contrasting light-colored crystals embedded in a darker, fine-grained matrix.
