igneous

Porphyritic Basalt (or Andesite) Pebble

Porphyritic Mafic Volcanic Rock

Hardness: 6-7 (Mohs scale); Color: Dark grey to greenish-grey matrix with white inclusions; Luster: Dull/Sub-vitreous; Structure: Porphyritic (two distinct grain sizes); Specific Gravity: 2.8-3.0.

Hardness: 6-7 (Mohs scale)
Color: Dark grey to greenish-grey matrix with white inclusions
Luster: Dull/Sub-vitreous

Identified March 20, 2026More igneous →

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Physical properties

Hardness: 6-7 (Mohs scale); Color: Dark grey to greenish-grey matrix with white inclusions; Luster: Dull/Sub-vitreous; Structure: Porphyritic (two distinct grain sizes); Specific Gravity: 2.8-3.0.

Formation & geological history

Formed from the rapid cooling of volcanic lava on the surface, where larger crystals (phenocrysts, likely plagioclase feldspar) formed slowly at depth before the eruption, while the fine-grained matrix cooled quickly upon eruption. Often smoothed into pebbles by river or beach erosion.

Uses & applications

Commonly used as aggregate in road construction, landscaping decor, and occasionally as a tumbling stone for hobbyists. Larger deposits are crucial for heavy-duty construction material.

Geological facts

The 'polka-dot' appearance is a classic example of a two-stage cooling process in geology. Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor.

Field identification & locations

Identify in the field by looking for a fine-grained, dark background with distinct, rectangular or lath-shaped white crystals. They are commonly found on Pacific Northwest beaches and in volcanic regions globally.

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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)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

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

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Magnetite

Magnetite (Fe3O4)

mineral