igneous

Amygdaloidal Basalt

Amygdaloidal Basalt (with secondary mineral infilling)

Hardness: 5-6 (basalt matrix); Color: Dark grey to greenish-grey matrix with orange, white, or brown vesicles; Luster: Dull to earthy; Texture: Vesicular/Porphyritic; Structure: Amygdules (gas bubbles filled with minerals like calcite, zeolites, or quartz).

Hardness: 5-6 (basalt matrix)
Luster: Dull to earthy

Identified April 6, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the cooling of extrusive volcanic lava. As the lava cools, gas bubbles are trapped, creating voids (vesicles). Over millions of years, mineral-rich groundwater flows through these holes, depositing secondary minerals that fill the cavities.

Uses & applications

Primarily of interest to geologists and mineral collectors. Basalt itself is used extensively in construction as crushed stone for roads and concrete aggregate.

Geological facts

The word 'amygdule' comes from the Greek word 'amygdala', meaning almond, referring to the almond-like shape of many of the hole infills. These rocks are common in the Lake Superior region and often contain native copper.

Field identification & locations

Identify by looking for a dark, heavy, fine-grained rock containing pockmarks or rounded 'spots' of a different mineral. Common in volcanic terrains like the Pacific Northwest, Hawaii, or the Keweenaw Peninsula.

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