igneous

Amygdaloidal Basalt

Amygdaloidal Basalt (containing secondary minerals like Quartz, Zeolites, or Calcite)

Hardness: 5-7 Mohs; Color: Dark grey to greenish-black groundmass with light-colored circular or almond-shaped inclusions; Luster: Dull (matrix) to vitreous (vesicle fill); Crystal structure: Aphanitic (fine-grained) matrix with amygdules.

Hardness: 5-7 Mohs
Luster: Dull (matrix) to vitreous (vesicle fill)

Identified March 7, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the cooling of mafic lava flows. As gas bubbles (vesicles) were trapped in the solidifying rock, they were later filled by secondary minerals precipitating from groundwater. Most common in ancient volcanic provinces (spanning from Precambrian to Cenozoic).

Uses & applications

Used primarily as decorative garden stones or aquarium decor. Exceptionally well-preserved specimens can be used for lapidary work (cabochons). Historically, when containing native copper (as in Michigan), used for metal extraction.

Geological facts

The name comes from the Greek word 'amygdala', meaning almond, referring to the shape of the mineral-filled gas bubbles. These rocks often trace the ancient migration path of hydrothermal fluids through volcanic fields.

Field identification & locations

Identify in the field by looking for a dark, heavy volcanic base pitted with lighter, rounded mineral 'spots' that don't look like standard crystals. Found commonly in Lake Superior (Michigan/Ontario), Iceland, and the Deccan Traps in India.

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