igneous

Porphyry

Porphyritic Igneous Rock (likely Porphyritic Andesite or Basalt)

Hardness: 6-7 Mohs; Color: Dark grey to black groundmass with lighter grey to greenish phenocrysts (crystals); Luster: Sub-vitreous to dull; Crystal structure: Aphanitic (fine-grained) base with phaneritic (visible) crystals; Specific gravity: 2.5-3.0.

Hardness: 6-7 Mohs
Luster: Sub-vitreous to dull

Identified April 11, 2026More igneous →

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Physical properties

Formation & geological history

Formed in a two-stage cooling process of magma: large crystals (phenocrysts) form slowly deep underground, and the remaining liquid cools rapidly upon eruption or rising, creating the fine-grained matrix. Often associated with volcanic arcs and subduction zones.

Uses & applications

Used primarily as decorative stone, construction aggregate, and in monuments or sculptures due to its interesting texture and durability. Historically prized by Romans for carvings.

Geological facts

The word 'Porphyry' comes from the Greek word for 'purple', as the most famous ancient specimens were deep red or purple. It was once reserved exclusively for imperial Roman use.

Field identification & locations

Identify in the field by looking for 'spots' (large crystals) embedded in a much finer-grained, solid-colored rock. These specimens are often found in glacially deposited gravel or near volcanic regions. For collectors, look for high contrast between the background and the crystals.

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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral