igneous

Porphyritic Basalt

Porphyritic Basalt (with plagioclase phenocrysts)

Hardness: 6.0 (Mohs); Color: Dark gray to black (mafic) groundmass with white/clear rectangular crystals; Luster: Dull/earthy matrix with vitreous crystals; Crystal structure: Porphyritic texture (aphanitic groundmass with visible phenocrysts); Cleavage: Distinct in phenocrysts; Specific gravity: 2.8–3.0.

Hardness: 6
Luster: Dull/earthy matrix with vitreous crystals

Identified March 30, 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 via two-stage cooling: slow underground cooling allows large phenocrysts (plagioclase) to form, followed by rapid eruption and cooling of the surrounding lava (groundmass). Commonly found in volcanic arcs and hotspots from any geological era.

Uses & applications

Primarily used as construction aggregate, road base, and ballast. High-quality specimens with dramatic contrast are used as decorative garden stone or lapidary slabs for bookends.

Geological facts

This texture is effectively a 'geological time capsule,' recording two distinct stages of a volcano's thermal history. It is a common component of oceanic crust and lunar maria.

Field identification & locations

Identify by looking for white, often lath-shaped or rectangular crystals (phenocrysts) embedded in a very fine-grained, dark-colored rock. Common in the Pacific Northwest, Hawaii, and Iceland. Collectors should look for sharp, unbroken crystals for best value.

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