igneous
Porphyritic Basalt (or Porphyry)
Porphyritic Andesite/Basalt (primarily Plagioclase feldspar and Pyroxene)
Hardness: 6.0-7.0 (Mohs); Color: Dark grey or greenish-grey groundmass with white phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic matrix with visible macro-crystals; Specific gravity: 2.8-3.0
- Hardness
- 6
- Color
- Dark grey or greenish-grey groundmass with white phenocrysts
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6.0-7.0 (Mohs); Color: Dark grey or greenish-grey groundmass with white phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic matrix with visible macro-crystals; Specific gravity: 2.8-3.0
Formation & geological history
Formed through a two-stage cooling process of volcanic magma: initially cooling slowly underground to form large crystals (phenocrysts), followed by rapid eruption and cooling of the remaining lava. Likely from the Cenozoic or Mesozoic era depending on the specific volcanic arc.
Uses & applications
Used primarily as crushed stone in construction, road base, and occasionally as decorative garden stones or lapidary slabs for bookends and tiles.
Geological facts
The term 'Porphyry' comes from the Greek word for purple, as the most famous ancient specimens prized by Roman emperors were deep purple. This specific 'salt and pepper' look is very common in volcanic island arcs.
Field identification & locations
Identify by looking for distinct, isolated white rectangular crystals (plagioclase) embedded in a fine-grained, darker stony background. It is commonly found in coastal areas, riverbeds near volcanic mountains, and tectonic plate boundaries.
