Amygdaloidal Basalt with Epidote

Physical Properties

Hardness: 6-7 (groundmass and epidote); Color: Dark grey to reddish-brown matrix with light green (pistachio) inclusions; Luster: Dull to earthy matrix, vitreous inclusions; Structure: Aphanitic with amygdaloidal textures.

Formation & Geological History

Formed from the rapid cooling of basaltic lava. Gas bubbles (vesicles) trapped during cooling were later filled with minerals like epidote, quartz, or chlorite from hydrothermal fluids, often during the Proterozoic era (approx. 1 billion years ago in regions like the Lake Superior Basin).

Uses & Applications

Primarily used as decorative stone, lapidary material for cabochons, and as a geological teaching specimen. Occasionally used in construction as crushed stone.

Geological Facts

These rocks are commonly known as 'Lakers' or 'Epidote stones' in the Great Lakes region. The green mineral, Epidote, gets its name from the Greek word 'epidosis' meaning 'addition'.

Field Identification & Locations

Identify in the field by looking for 'pistachio green' spots inside dark, heavy volcanic rock. Commonly found on beaches of Lake Superior, Michigan, and in volcanic provinces worldwide.

Identified on: 4/25/2026

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