igneous

Amygdaloidal Basalt

Amygdaloidal Basalt (Mafic Extrusive volcanic rock)

Hardness: 6-7 (Mohs/Basalt matrix), Color: Dark grey to reddish-brown groundmass with white/clear mineral fillings, Luster: Dull/Earthy matrix with vitrous/silky amygdules, Texture: Porphyritic or fine-grained with almond-shaped gas cavities (vesicles) filled by secondary minerals.

Identified April 12, 2026More igneous →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium and iron-rich lava. As the lava cools, gas bubbles are trapped (vesicles). Over geological time, groundwater Richter in dissolved minerals flows through the rock, depositing minerals like quartz, calcite, or zeolites into the bubbles, turning them into 'amygdules.'

Uses & applications

Used primarily in construction as crushed stone or aggregate. High-quality specimens with colorful mineral fillings (like agates) are popular for rock collecting and lapidary work.

Geological facts

The word 'amygdule' comes from the Latin word for 'almond,' referring to the shape of the mineral-filled gas pockets. The Lake Superior region is famous for amygdaloidal basalts that contain native copper.

Field identification & locations

Identify in the field by look for a heavy, dark-colored rock with distinct 'spots' or 'pockmarks' that are filled with a different, lighter mineral. Common in volcanic fields and oceanic crust exposures.

Other igneous specimens

See all

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