igneous

Amygdaloidal Basalt

Amygdaloidal Basalt (Mafic volcanic rock)

Hardness: 5-6 (Mohs scale); Color: Dark grey, black, or reddish-brown matrix with white, clear, or green infillings; Luster: Dull matrix with vitreous (glassy) or pearly vugs; Structure: Vesicular/Amygdaloidal (pitted with filled holes); Specific Gravity: 2.8 - 3.0.

Hardness: 5-6 (Mohs scale)
Luster: Dull matrix with vitreous (glassy) or pearly vugs

Identified March 28, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava (basalt). During cooling, gas bubbles leave cavities (vesicles). Over millions of years, groundwater deposits minerals like quartz, calcite, or zeolites into these holes, turning them into 'amygdules.'

Uses & applications

Used primarily in construction as crushed stone or aggregate. High-quality specimens with semi-precious infillings (like agate) are used for jewelry, lapidary work, and geological collections.

Geological facts

The word 'amygdule' comes from the Greek word for 'almond,' describing the shape of the mineral infillings. This rock is a primary source for Native American copper artifacts in the Lake Superior region, where native copper often fills the cavities.

Field identification & locations

Identify by looking for a heavy, dark, fine-grained rock with distinct pits or 'pockets' containing different colored minerals. Commonly found in volcanic regions such as the Deccan Traps (India), the Giant’s Causeway (Ireland), and the Lake Superior North Shore (USA/Canada).

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