igneous

Amygdaloidal Basalt

Amygdaloidal Basalt (Mafic Extrusive volcanic rock)

Hardness: 6-7 (Mohs scale); Color: Dark grey to black groundmass with light green, white, or blue mineral inclusions; Luster: Dull/earthy matrix with vitreous inclusions; Structure: Aphanitic (fine-grained) with amygdules (mineral-filled vesicles).

Hardness: 6-7 (Mohs scale)
Luster: Dull/earthy matrix with vitreous inclusions

Identified March 21, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium and iron-rich lava. Gas bubbles (vesicles) trapped in the cooling lava are later filled with secondary minerals such as chlorite, epidote, or quartz through hydrothermal precipitation, often in Precambrian or more recent volcanic environments.

Uses & applications

Commonly used in construction as crushed stone or aggregate. High-quality specimens with colorful mineral infills like epidote or copper are prized by rockhounds and mineral collectors.

Geological facts

The term 'amygdule' comes from the Greek word 'amygdala' meaning almond, referring to the almond-shaped mineral deposits. In regions like Lake Superior, these rocks can contain native copper or rare zeolites.

Field identification & locations

Identify in the field by looking for a dark, heavy rock with small, rounded, differently colored 'spots' or 'pitted' surfaces where minerals have weathered out. Common in volcanic rift zones and shorelines like 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