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 In gemology, chatoyancy ( ), or chatoyance or cat's eye effect, is an optical reflectance effect seen in certain gemstones, woods, and carbon fibre. Coined from the French "œil de chat", meaning "cat's eye", chatoyancy arises either from the ...

, meaning they have a varying luster. This chatoyancy can be subtranslucent to opaque. Cholorastrolite is a variety of

pumpellyite Pumpellyite is a group of closely related sorosilicate minerals: *pumpellyite-(Mg): *pumpellyite-(Fe2+): *pumpellyite-(Fe3+): *pumpellyite-(Mn2+): *pumpellyite-(Al): Pumpellyite crystallizes in the monoclinic-prismatic crystal system. It ty ...

: . Chlorastrolite was once thought to be an impure variety of

prehnite Prehnite is an Silicate minerals, inosilicate of calcium and aluminium with the formula: Ca2Al(AlSi3O10)(OH)2. Limited Fe3+ substitutes for aluminium in the structure. Prehnite crystallizes in the orthorhombic crystal system, and most often forms a ...

thomsonite Thomsonite is the name of a series of tecto-silicate minerals of the zeolite group. Prior to 1997, thomsonite was recognized as a mineral species, but a reclassification in 1997 by the International Mineralogical Association changed it to a ser ...

Occurrences

Chlorastrolite occurs as amygdaloid structures and fracture fillings in

basalt Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90 ...

, 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 of the

Upper Peninsula of Michigan The Upper Peninsula of Michigan – also known as Upper Michigan or colloquially the U.P. – 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 by ...

and

Isle Royale Isle Royale National Park is an American national park consisting of Isle Royale – known as Minong to the native Ojibwe – along with more than 400 small adjacent islands and the surrounding waters of Lake Superior, in the state of Michigan. ...

Lake Superior Lake Superior in central North America is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. and the third-largest by volume, holding 10% of the world's surface fresh wa ...

. 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 U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest, upper Midwestern United States. With a population of nearly 10.12 million and an area of nearly , Michigan is the List of U.S. states and ...

Other Names

*''Green starstone'' is most common in

Upper Peninsula The Upper Peninsula of Michigan – also known as Upper Michigan or colloquially the U.P. – 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 by ...

. *''Greenstone'' or ''

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 off the coast of

Scotland Scotland (, ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a Anglo-Scottish border, border with England to the southeast ...

. *''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 Nipigon () is a township in Thunder Bay District, Northwestern Ontario, Canada, located along the west side of the Nipigon River and south of the small Lake Helen running between Lake Nipigon and Lake Superior. Lake Nipigon is located approximat ...

(on

Ontario Ontario ( ; ) is one of the thirteen provinces and territories of Canada.Ontario is located in the geographic eastern half of Canada, but it has historically and politically been considered to be part of Central Canada. Located in Central C ...

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

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

References

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 {{silicate-mineral-stub