mineral

Chlorastrolite (Isle Royale Greenstone)

Chlorastrolite (Pumpellyite-(Mg,Fe2+): Ca2MgAl2(Si2O7)(SiO4)(OH)2∙H2O)

AI-generated identification · may be incorrect

Hardness: 5.5 to 6.0 on Mohs scale; Color: Mottled green, bluish-green with dark matrix; Luster: Silky to pearly in polished state; Crystal structure: Monoclinic; Cleavage: Good; Specific gravity: 3.2-3.4.

Hardness: 5
Color: Mottled green, bluish-green with dark matrix
Luster: Silky to pearly in polished state

Identified March 24, 2026More mineral →

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Physical properties

Formation & geological history

Formed as secondary mineral fillings (amygdules) within the gas cavities of basaltic lava flows. Specifically localized in the Keweenawan volcanic rocks of the Upper Peninsula of Michigan, dating back approximately 1.1 billion years.

Uses & applications

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

Geological facts

Also known as 'Green Star Stone' because of its unique radiated, turtle-shell-like pattern. Specimens are increasingly rare as it can only be legally collected in specific locations outside of Isle Royale National Park, where collection is prohibited.

Field identification & locations

Identify by its distinctive 'turtle-back' pattern of light and dark green radiating crystals. Can be found on the beaches of the Keweenaw Peninsula, Michigan, especially after storms. Search for dark, pebble-shaped stones with hints of green radial patterns.