igneous

Porphyritic Basalt (with Amygdales)

Amygdaloidal Porphyritic Basalt

Hardness: 5-6 (Mohs); Color: Dark brown to reddish-purple groundmass with white/clear mineral inclusions; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained) matrix with visible phenocrysts/vesicle fillings; Specific Gravity: 2.8–3.0

Hardness: 5-6 (Mohs)
Luster: Dull to sub-vitreous

Identified April 5, 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 'holes' (vesicles) were formed by gas bubbles trapping in cooling lava, which later filled with secondary minerals like quartz, calcite, or zeolites through hydrothermal activity.

Uses & applications

Commonly used as crushed stone for aggregate in construction, road base, and railroad ballast. Decorative specimens are used in landscaping or small-scale geology collections.

Geological facts

The reddish-purple hue suggests high iron content that has undergone oxidation (weathering). These rocks often record the volatile content of ancient volcanic eruptions through their gas-bubble patterns.

Field identification & locations

Identify in the field by looking for a dense, dark, fine-grained rock containing distinct small white or translucent 'spots' that don't look like regular rounded pebbles (as in conglomerate) but rather secondary mineral growths. Frequently found in volcanic fields like the Deccan Traps or the Lake Superior region.

Other igneous specimens

See all

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