mineral
Chlorastrolite (Isle Royale Greenstone)
Chlorastrolite (Pumpellyite-(Mg,Fe2+): Ca2MgAl2(Si2O7)(SiO4)(OH)2∙H2O)
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
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Physical properties
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.
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.
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Sandstone (with potential mineral staining/concretions)
Arenite (SiO2 based)
sedimentary
Epidote
Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)
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Nephrite Jade
Nephrite
Mineral/Rock
Banded Gneiss (River Rock/Pebble)
Gneiss (specifically a banded variety often found as a river rock)
Metamorphic Rock
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Green Apatite on Albite
Fluorapatite [Ca5(PO4)3F] on Albite [Na(AlSi3O8)]
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Epidote
Epidote - Ca2(Al2,Fe3+)3(SiO4)3(OH)
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