mineral

Chlorastrolite (Greenstone)

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

Hardness: 5 to 6 on Mohs scale; Color: typically green to dark green with white or yellowish patterns; Luster: Pearly to dull; Crystal structure: Monoclinic; Cleavage: Good; Specific gravity: 3.2

Hardness: 5 to 6 on Mohs scale
Luster: Pearly to dull

Identified March 28, 2026More mineral →

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Physical properties

Hardness: 5 to 6 on Mohs scale; Color: typically green to dark green with white or yellowish patterns; Luster: Pearly to dull; Crystal structure: Monoclinic; Cleavage: Good; Specific gravity: 3.2

Formation & geological history

Forms as a secondary mineral in the amygdules (gas bubbles) of basaltic lava flows. It is chemically deposited by hydrothermal fluids during the cooling and weathering of volcanic rock. The specimens from Isle Royale are approximately 1.1 billion years old (Precambrian).

Uses & applications

Primarily used as a gemstone for jewelry (cabochons), as well as a popular collector's mineral and lapidary material. It is the official state gem of Michigan.

Geological facts

Known as the "Isle Royale Greenstone," it features a unique 'turtle-shell' pattern caused by radiating fibrous crystals. Only the highest quality stones show the chatoyancy (cat's-eye effect) that makes it prized. It is extremely rare outside of the Lake Superior region.

Field identification & locations

Identify by its dark green base and distinct radiating, mosaic, or 'turtle-back' pattern of lighter green/white. Found primarily on Isle Royale (collecting is prohibited there) and in the Keweenaw Peninsula of Michigan. Collectors look for small, pebble-like nodules on beaches or in mine tailings.

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