igneous

Porphyritic Basalt

Porphyritic Basalt (with Plagioclase phenocrysts)

Hardness: 6-7 on Mohs scale; Color: Dark grey to black groundmass with light-colored (white/creamy) crystals; Luster: Dull/earthy matrix with vitreous crystals; Crystal Structure: Porphyritic (fine-grained matrix with larger visible crystals); Cleavage: Distinct in phenocrysts; Specific Gravity: 2.8-3.0.

Hardness: 6-7 on Mohs scale
Luster: Dull/earthy matrix with vitreous crystals

Identified March 30, 2026More igneous →

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Physical properties

Formation & geological history

Formed through a two-stage cooling process of mafic lava: first, slow cooling deep underground allows large crystals (phenocrysts) to grow; second, a rapid volcanic eruption causes the remaining liquid to cool quickly into a fine-grained matrix. Found in volcanic regions of all geological ages, especially near mid-ocean ridges and hotspots.

Uses & applications

Used in construction as crushed stone or aggregate for roads and concrete. Often used as high-quality 'railroad ballast.' Higher-quality decorative specimens are used in landscaping and as educational geological samples for collectors.

Geological facts

The word 'basalt' is derived from a Latin word for 'very hard rock.' Basalt is the most common rock type on the Earth's surface and also makes up most of the surface of the Moon and Mars.

Field identification & locations

Identify in the field by looking for a 'salt and pepper' appearance where larger white crystals are embedded in a dark, dense, heavy rock. Common in Hawaii, Iceland, and the Pacific Northwest. Collectors should look for samples where the phenocrysts are sharp, well-defined, and show clear rectangular or tabular shapes.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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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

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Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral