Chlorastrolite, also known as Isle Royale Greenstone, is a green or bluish green stone.Arguably the rarest gemstone in the world and one of the most valuable American gemstones. The gem quality specimens are typically found on Isle Royale, a large island off the Northeastern tip of Minnesota.
Michigan's State Gemstone
/ref> Chlorastrolite has finely radiating or stellate (for examples, see crystal habits) masses that have a "turtleback" pattern. The stellate masses tend to be chatoyant, meaning they have a varying luster. This chatoyancy can be subtranslucent to opaque. Cholorastrolite is a variety of pumpellyite: . Chlorastrolite was once thought to be an impure variety of prehnite or thomsonite.

Occurrences

Chlorastrolite occurs as amygdaloid structures and fracture fillings in

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

, and when the water and wave action has worn away the basalt, they are found as beach pebbles and granules in loose sediments. It is found in the

Keweenaw Peninsula The Keweenaw Peninsula (, ) is a peninsula of the U.S. state of Michigan. Part of the greater landmass of the Upper Peninsula of Michigan, Upper Peninsula, the Keweenaw Peninsula projects about northeasterly into Lake Superior, forming Keweena ...

of the

Upper Peninsula of Michigan The Upper Peninsula of Michigan—also known as Upper Michigan or colloquially the U.P. or Yoop—is the northern and more elevated of the two major landmasses that make up the U.S. state of Michigan; it is separated from the Lower Peninsula of ...

and Isle Royale in

Lake Superior Lake Superior is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. Lake Michigan–Huron has a larger combined surface area than Superior, but is normally considered tw ...

. Isle Royale is a National Park, and so it is illegal to collect specimens there. It is difficult to identify an unpolished pebble of chlorastrolite. Most gem quality chlorastrolite stones are very small, and it is rare to find one that is larger than a half inch. The largest gem quality stone is in the Smithsonian Museum and measures 1.5 by 3 inches. It was first described from Isle Royale, Lake Superior by C. T. Jackson and J. D. Whitney in 1847. Chlorastrolite, also known as greenstone, is the official

state gem Leaders of states in the U.S. which have significant mineral deposits often create a state mineral, rock, stone or gemstone to promote interest in their natural resources, history, tourism, etc. Not every state has an official state mineral, rock, ...

Michigan Michigan ( ) is a peninsular U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest, Upper Midwestern United States. It shares water and land boundaries with Minnesota to the northwest, Wisconsin to the west, ...

Other names

*''Green starstone'' is most common in

's Upper Peninsula. *''Greenstone'' or '' Isle Royale greenstone'' and ''

greenstone''. *''Turtle back'' is a name sometimes given because of the overall pattern, particularly in polished pieces, which roughly resembles that of a turtle shell. *''Uigite'' is a variety found on the island of

Skye The Isle of Skye, or simply Skye, is the largest and northernmost of the major islands in the Inner Hebrides of Scotland. The island's peninsulas radiate from a mountainous hub dominated by the Cuillin, the rocky slopes of which provide some o ...

off the coast of

Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...

. *''Zonochlorite'' is a misnomer sometimes given rounded masses of chlorastrolite within or weathered out of amygdaloidal cavities, especially those from the area around Nipigon Bay (on

Ontario Ontario is the southernmost Provinces and territories of Canada, province of Canada. Located in Central Canada, Ontario is the Population of Canada by province and territory, country's most populous province. As of the 2021 Canadian census, it ...

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

. Confusion may derive from the fact that zonochlorite is sold as faux ''chlorastrolite''.

References

Sources

R.V.Dietrich: ''Gemrocks - chlorastrolite''
* ttp://www.mindat.org/min-11469.html Mindat w/ locations Calcium minerals Magnesium minerals Iron(II) minerals Aluminium minerals Silicate minerals Gemstones Lake Superior Geology of Michigan Symbols of Michigan Isle Royale National Park {{gemology-stub