igneous

Porphyry

Porphyritic Basalt/Andesite

Hardness: 6-7 on Mohs scale; Color: Dark grey to black groundmass with white to off-white phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic matrix with large visible crystals (phenocrysts).

Hardness: 6-7 on Mohs scale
Luster: Dull to sub-vitreous

Identified April 6, 2026More igneous →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed through a two-stage cooling process of magma: slow cooling at depth creates large crystals, followed by rapid eruption and cooling of the remaining melt. Often associated with volcanic arcs and tectonic plate boundaries.

Uses & applications

Used primarily in construction as crushed stone or road ballast. Historically used as a decorative building stone and for monuments in Ancient Rome.

Geological facts

The word 'porphyry' comes from the Greek word for 'purple.' While many are grey/black, the most famous historical porphyry was Imperial Purple Porphyry from Egypt, reserved exclusively for Emperors.

Field identification & locations

Identify by looking for 'spots' or 'stars' (large distinct crystals) embedded in a much finer, uniform background grain. This texture signifies a complex cooling history.

Other igneous specimens

See all

Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous