Rock Identifier
Chlorastrolite (Isle Royale Greenstone) (Chlorastrolite (Pumpellyite-(Mg,Al)), formula: Ca2(Mg,Fe)Al2(SiO4)(Si2O7)(OH)2∙H2O) — mineral
mineral

Chlorastrolite (Isle Royale Greenstone)

Chlorastrolite (Pumpellyite-(Mg,Al)), formula: Ca2(Mg,Fe)Al2(SiO4)(Si2O7)(OH)2∙H2O

Hardness: 5.5–6.0 on Mohs scale; Color: Dark green with lighter green 'turtle-back' patterns; Luster: Vitreous to silky (pearly when polished); Crystal structure: Monoclinic; Specific gravity: 3.1–3.5; Cleavage: Good in two directions.

Hardness
5
Color
Dark green with lighter green 'turtle-back' patterns
Luster
Vitreous to silky (pearly when polished)
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Physical properties

Hardness: 5.5–6.0 on Mohs scale; Color: Dark green with lighter green 'turtle-back' patterns; Luster: Vitreous to silky (pearly when polished); Crystal structure: Monoclinic; Specific gravity: 3.1–3.5; Cleavage: Good in two directions.

Formation & geological history

Formed as amygdaloid (gas cavity) fillings in basaltic lava flows, specifically the Keweenawan volcanic series of the Mid-continent Rift System. It formed approximately 1.1 billion years ago during the Mesoproterozoic era.

Uses & applications

Predominantly used as a gemstone in jewelry (cabochons) and a highly prized collector's mineral. It is the official state gem of Michigan.

Geological facts

Also known as 'Green Star Stone' because of its unique chatoyant, star-like patterns. The name Chlorastrolite is derived from Greek words meaning 'green star stone.' Authentic specimens are primarily found on Isle Royale in Lake Michigan and Michigan's Keweenaw Peninsula.

Field identification & locations

Identify in the field by its distinct 'turtle-back' or radiating acicular pattern. It is usually found as small, rounded pebbles weathered out of basalt or embedded in the shoreline rocks. Look for dark green stones with light green web-like fractures.

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