igneous

Porphyritic Basalt

Porphyritic Basalt (Mafic extrusive rock)

Hardness: 6-7 (Mohs); Color: Dark grey to black groundmass with light-colored white/cream phenocrysts; Luster: Dull to sub-vitreous; Texture: Porphyritic (fine-grained matrix with larger visible crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0.

Hardness: 6-7 (Mohs)
Luster: Dull to sub-vitreous

Identified March 20, 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 from the cooling of magnesium-rich and iron-rich lava. The large crystals (phenocrysts), typically plagioclase feldspar, formed slowly underground before a volcanic eruption caused the remaining liquid lava to cool quickly on the surface, creating the fine-grained matrix.

Uses & applications

Commonly used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Select pieces are used as decorative garden stones or for lapidary work like pavers and tiles.

Geological facts

Basalt is the most common rock type on Earth's crust and also makes up most of the ocean floors. Porphyritic textures are significant because they record two distinct stages of cooling in a single volcanic event.

Field identification & locations

Identify in the field by looking for a 'salt and pepper' look where white rectangular crystals are embedded in a dark, dense, heavy rock. Found globally in volcanic fields like the Pacific Northwest (USA), Iceland, and India's Deccan Traps.

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