igneous

Basalt (Amygdaloidal)

Amygdaloidal Basalt

Hardness: 5-6 (Mohs scale); Color: Dark grey to black matrix with lighter colored mineral inclusions; Luster: Dull (matrix) to vitreous (vesicle fills); Structure: Aphanitic with vesicles; Cleavage: None.

Hardness: 5-6 (Mohs scale)
Luster: Dull (matrix) to vitreous (vesicle fills)

Identified February 9, 2026More igneous →

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Physical properties

Formation & geological history

Formed from extrusive volcanic activity where gas bubbles (vesicles) are created as lava cools. Over time, secondary minerals like calcite, quartz, or zeolites leach into these cavities and crystallize. These can range from recent to billions of years old.

Uses & applications

Common basalt is used in road construction as aggregate. Distinctive amygdaloidal specimens are primarily collected by hobbyists or used in lapidary work as decorative stones or cabochons.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. When the gas bubbles in volcanic rock are filled with minerals such as Agate or Zeolite, it is called 'amygdaloidal' from the Greek word for almond.

Field identification & locations

Identify by its heavy weight, dark fine-grained base, and the presence of small, contrasting mineral 'spots' or 'pockets'. This specimen appears to be water-worn (tumbled), common on beaches or in riverbeds near volcanic regions like the Lake Superior area.

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