igneous
Porphyry
Porphyritic Andesite/Basalt
Hardness: 6-7 (Mohs scale), Color: Dark grey or brown matrix with light grey to white phenocrysts, Luster: Dull/Sub-vitreous, Crystal structure: Aphanitic matrix with visible crystals (phenocrysts).
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Dark grey or brown matrix with light grey to white phenocrysts, Luster: Dull/Sub-vitreous, Crystal structure: Aphanitic matrix with visible crystals (phenocrysts).
Formation & geological history
Formed from molten rock cooling in two stages; first cooling slowly deep underground to form large crystals, followed by a rapid eruption and cooling of the remaining magma.
Uses & applications
Primarily used as crushed stone in construction, road base, and occasionally as decorative stone or for lapidary tumbling due to its contrasting patterns.
Geological facts
The word 'porphyry' comes from the Ancient Greek word for purple, as the most famous ancient specimens were deep purple. It was highly prized by Roman emperors for monumental architecture.
Field identification & locations
Identify by looking for a fine-grained, solid background rock embedded with distinct, visible square or rectangular crystals. Commonly found in volcanic regions or as river-worn cobbles.
