mineral

Chlorastrolite (Greenstone)

Chlorastrolite (Pumpellyite variety), Ca2(Mg,Fe)Al2(SiO4)(Si2O7)(OH)2·H2O

Hardness: 5.5-6.0; Color: Dark to light green with unique 'turtleback' radiating patterns; Luster: Pearly to silky; Crystal Structure: Monoclinic; Cleavage: Good in one direction; Specific Gravity: 3.1-3.5.

Hardness: 5
Luster: Pearly to silky

Identified March 22, 2026More mineral →

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Physical properties

Formation & geological history

Formed as secondary mineral fillings (amygdules) within gas bubbles of basaltic lava flows. Most deposits are approximately 1.1 billion years old (Precambrian), associated with the Midcontinent Rift System.

Uses & applications

Primarily used as a gemstone for jewelry (cabochons) and a highly sought-after specimen for mineral collectors. It is the official state gem of Michigan.

Geological facts

Commonly known as 'Isle Royale Greenstone,' it is found almost exclusively in the Lake Superior region. The name 'Chlorastrolite' translates from Greek to 'green star stone,' referring to its radiating fibrous pattern.

Field identification & locations

Identify in the field by its distinct 'turtleback' or 'star' pattern and green color. It is most commonly found on the beaches of Isle Royale and Michigan's Keweenaw Peninsula. Collecting on Isle Royale is strictly prohibited, so it is usually searched for on mainland beaches.

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