igneous

Amygdaloidal Basalt

Vesicular-Amygdaloidal Basalt (Mafic Extrusive)

Hardness: 5-6 (Mohs); Color: Dark grey, reddish-brown, or purple-maroon groundmass with white, green, or pink mineral spots; Luster: Dull/Earthy (groundmass), Vitreous (fillings); Structure: Aphanitic with rounded amygdules.

Hardness: 5-6 (Mohs)
Luster: Dull/Earthy (groundmass), Vitreous (fillings)

Identified April 3, 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 gas bubbles trapped in cooling lava (vesicles). Later, mineral-rich groundwater flows through the rock, depositing minerals like quartz, calcite, or zeolites in the voids. Commonly found in Mesozoic or Precambrian volcanic flood deposits.

Uses & applications

Commonly used as high-grade road base or concrete aggregate. Polished specimens are used in the lapidary arts for jewelry, particularly those from the Lake Superior region containing copper or agates.

Geological facts

The spots in the rock are called amygdules, derived from the Greek word 'amygdalē' meaning almond. In the Keweenaw Peninsula, these cavities are often filled with native copper, creating some of the richest copper deposits in history.

Field identification & locations

Identify by looking for a fine-grained, dark igneous matrix containing distinct, rounded 'eye-like' structures filled with secondary minerals. Found in basaltic lava flow areas.

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

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral