igneous

Amygdaloidal Basalt

Amygdaloidal Basalt (extrusive volcanic rock with mineral inclusions)

Hardness: 6-7 (groundmass), 3-7 (vesicle fillings); Color: Dark brown to dark gray matrix with light-colored (white/green/pink) circular spots; Luster: Dull to earthy matrix, vitreous to silky fillings; Structure: Porphyritic or vesicular; Specific gravity: 2.8-3.0.

Hardness: 6-7 (groundmass), 3-7 (vesicle fillings)
Luster: Dull to earthy matrix, vitreous to silky fillings

Identified March 21, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the cooling of gas-rich lava. As the lava solidifies, gas bubbles are trapped, creating cavities (vesicles). Later, mineral-rich groundwater flows through, depositing minerals like quartz, calcite, or zeolites into the holes. Most specimens are from the Cenozoic or Mesozoic eras.

Uses & applications

Commonly used as lapidary material for polishing and making jewelry (cabochons), as well as decorative garden stones and road aggregate in areas where it is abundant.

Geological facts

The name 'Amygdaloid' comes from the Greek word for 'almond' because the mineral-filled cavities often resemble almond seeds in shape and size. These rocks are frequently indicators of ancient volcanic activity near water sources.

Field identification & locations

Identify by looking for a dark, fine-grained rock containing distinct rounded 'spots' of different minerals. Commonly found in basaltic flow regions like Lake Superior (USA), the Deccan Traps (India), and Iceland. To field test, check if the white spots react with acid (calcite) or are scratch-resistant (quartz).

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

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral