igneous

Porphyritic Basalt (Amygdaloidal)

Porphyritic Mafic Volcanic Rock (Amygdaloidal Basalt)

Hardness: 5-6 (Mohs scale); Color: Dark reddish-brown (russet) matrix with white phenocrysts/inclusions; Luster: Dull to earthy (matrix), vitreous to pearly (inclusions); Structure: Porphyritic and vesicular; Specific Gravity: 2.8-3.0.

Hardness: 5-6 (Mohs scale)
Luster: Dull to earthy (matrix), vitreous to pearly (inclusions)

Identified March 21, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The white spots are amygdules (secondary minerals like quartz, calcite, or zeolites) that filled gas bubbles (vesicles) after the rock solidified. Common in regions with ancient volcanic activity like the Lake Superior basin or the Pacific Northwest.

Uses & applications

Commonly used as crushed stone in construction, road base, and railroad ballast. Smooth river or beach-tossed specimens are popular in rock tumbling and landscaping.

Geological facts

This specific reddish hue often indicates 'oxidation' of iron minerals within the basalt, sometimes referred to as 'ferruginous basalt.' When the white inclusions are large and distinct, these are colloquially known as 'snowflake' or 'polka-dot' rocks by collectors.

Field identification & locations

Identify in the field by its distinctive 'spotted' appearance against a dark, fine-grained background and its heavy weight relative to its size. Commonly found on volcanic beaches and in glacial till in the Northern US and Canada. Often confused with porphyry or certain types of rhyolite.

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