igneous

Porphyritic Basalt

Porphyritic Basalt (with Plagioclase phenocrysts)

Hardness: 6-7 on Mohs scale; Color: Dark grey to black groundmass with light-colored (white/cream) spots; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic (fine-grained) matrix with large visible crystals (phenocrysts); Cleavage: Indistinct; Specific Gravity: 2.8-3.0.

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

Identified February 17, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the cooling of mafic lava. The large crystals (phenocrysts) formed slowly underground before the remaining lava erupted and cooled rapidly on the surface. This specific texture is called 'porphyritic'. Often associated with volcanic arcs and oceanic crust.

Uses & applications

Used primarily as crushed stone for construction aggregate, road base, and railroad ballast. Exceptionally attractive specimens are sometimes tumbled for decor or used as 'lucky stones' in pockets.

Geological facts

This texture is sometimes referred to as 'Chinese Writing Stone' when the crystals are needle-like, or 'Snowflake Stone'. The larger crystals are most commonly plagioclase feldspar.

Field identification & locations

Identify in the field by the distinct 'spotted' appearance of light crystals against a dark, dense matrix. Commonly found in areas of past volcanic activity, such as the Pacific Northwest, Hawaii, or glacial till in the Great Lakes region.

Other igneous specimens

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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