igneous

Porphyritic Basalt (with Augite Crystals)

Porphyritic Mafic Volcanic Rock (Fe-Mg Silicates)

Hardness: 5-7 (Mohs). Color: Dark gray to black matrix with darker, chunky phenocrysts. Luster: Dull to sub-vitreous. Structure: Porphyritic (large crystals in a fine-grained 'groundmass'). Cleavage: Poorly visible in matrix, two directions at nearly 90 degrees in crystals.

Hardness: 5-7 (Mohs)
Color: Dark gray to black matrix with darker, chunky phenocrysts
Luster: Dull to sub-vitreous

Identified March 7, 2026More igneous →

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Physical properties

Formation & geological history

Formed from volcanic activity where magma begins cooling slowly underground (forming the larger crystals) and then erupts, cooling rapidly on the surface (forming the fine-grained matrix). Often found in Cenozoic volcanic fields.

Uses & applications

Used primarily in road construction as crushed aggregate, as railroad ballast, and occasionally as decorative garden stones. High-quality specimens are used by geology students for studying igneous textures.

Geological facts

The 'porphyritic' texture is a geologic 'diary' of two-stage cooling: a slow subterranean stage followed by a fast eruptive stage. These rocks are common on the ocean floor and in volcanic island chains like Hawaii.

Field identification & locations

Identify in the field by looking for a 'chocolate chip cookie' appearance—dark, distinct crystal shapes embedded in a uniform, fine-grained background. Commonly found in coastal or volcanic regions like the Pacific Northwest or Scotland.

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