igneous

Amygdaloidal Basalt with Epidote/Chlorite

Basalt (containing (Ca2(Al2Fe3+)(SiO4)(Si2O7)O(OH)))

Hardness: 6-7 (epidote) vs 5-6 (basalt matrix). Color: Dark gray/black matrix with pistachio-green or chartreuse secondary mineral infills. Luster: Dull/earthy for matrix, vitreous for crystals. Structure: Fine-grained aphanitic matrix with irregular crystal-lined pockets (amygdules).

Hardness: 6-7 (epidote) vs 5-6 (basalt matrix)
Luster: Dull/earthy for matrix, vitreous for crystals

Identified April 11, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of basaltic lava. As gas bubbles (vesicles) were trapped in the cooling lava, they were later filled by hydrothermal fluids depositing secondary minerals like epidote, quartz, or chlorite during low-grade metamorphism.

Uses & applications

Mainly used as a decorative specimen for collectors. Basalt itself is used in construction as crushed stone for road base or concrete aggregate.

Geological facts

The green mineralization is often a result of hydrothermal alteration. When these vesicles are filled with minerals like epidote, the rock is specifically called an 'amygdaloid.' Such rocks are common in the Keweenawan Peninsula of Michigan.

Field identification & locations

Identify in the field by looking for 'pitted' or 'bubble-filled' dark volcanic rocks where the bubbles are filled with green, white, or clear crystals. It is a key indicator of past volcanic activity and hydrothermal fluid movement.

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