igneous

Amygdaloidal Basalt

Amygdaloidal Basalt (extrusive volcanic rock with secondary mineral infilling)

Hardness: 6-7 (Mohs scale); Color: Dark grey to greenish-black matrix with light green or white spots; Luster: Dull/earthy matrix with vitreous inclusions; Structure: Vesicular texture filled with secondary minerals; Cleavage: None (conchoidal fracture).

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 cooling of gas-rich lava flows. As gas bubbles (vesicles) are trapped in the cooling basalt, they later become filled with secondary minerals like epidote, quartz, or zeolites through groundwater infiltration. This specimen is likely from the Keweenawan age (approx. 1.1 billion years ago).

Uses & applications

Commonly used as decorative garden stones, for aquarium decor, or as cabochons in lapidary work. In smaller sizes, it is a popular specimen for educational geology kits.

Geological facts

The word 'amygdaloidal' comes from the Greek 'amygdala,' meaning almond, referring to the almond-shaped mineral deposits. This specific greenish variety is often associated with the 'Greenstone' (Chlorastrolite) found in Michigan's Lake Superior region.

Field identification & locations

Identify in the field by looking for dark, heavy volcanic rocks with distinct 'polka-dot' mineral fillings. Common in volcanic regions and glacial drifts, particularly around the Great Lakes and Pacific Northwest. Collectors often look for ones where the 'eyes' are bright green epidote.

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

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Magnetite

Magnetite (Fe3O4)

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