Igneous

Amygdaloidal Basalt

Amygdaloidal Basalt (Mafic volcanic rock including secondary mineral infills)

Hardness: 5-7 (groundmass), Color: dark grey to black matrix with white, green, or pink circular spots, Luster: dull to earthy matrix with vitreous inclusions, Structure: Vesicular/Porphyritic, Cleavage: None

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the cooling of magnesium-rich and iron-rich lava. Gases trapped in the lava create bubbles (vesicles), which are later filled by secondary minerals like quartz, calcite, or zeolites precipitated from groundwater.

Uses & applications

Used primarily in construction as crushed stone or aggregate. Historically used for carving. Attractive specimens are used for educational purposes and lapidary cutting.

Geological facts

The word 'Amygdaloidal' comes from 'amygdala', Latin for almond, referring to the almond-shaped mineral inclusions. These rocks are primary sources for high-quality agate and zeolites.

Field identification & locations

Identify by looking for a dark, dense rock base filled with distinct, rounded 'spots' or 'pebbles' of a different color. Common in volcanic regions like the Lake Superior basin or the Deccan Traps.

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