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September 2012 - Chlorastrolite Greenstone


The mineral of the month is Chlorastrolite, also known as Isle Royale greenstone. This mineral was designated as the official state gem by Michigan’s 76th Legislature (Act 56, PA 1972). This legislation was introduced by Representative Russell Hellman of Dollar Bay. This mineral is a green variety of the mineral pumpellyite, and has a pattern of star-like crystals which forms a turtle-shell pattern. Greenstones are found in the waters and on the shores of Isle Royale, where it is illegal to collect due to the island’s national park status. It can also be found in the Michigan Copper ranges in the western Upper Peninsula. Many of the old tailings left over from the mining era have greenstones in them. They appear as dark green, small round or almond shape nodules in the basaltic rock. In some cases these nodules have weathered out of the volcanic rock and can be found along the shoreline. Unfortunately, only a small percentage of the green nodules are actual greenstones, as most are chloride, prehnite or some other mix of minerals. And of course, of all the greenstones found, perhaps only a few out of every hundred are gem quality.

It is difficult to identify an unpolished pebble of chlorastrolite. Most gem quality greenstones 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 and measures 1.5 by 3 inches.


Since it is illegal to search for these gemstones on Isle Royal, the best localities now are the waste rock piles located in the old Keweenaw Peninsula mines in the western Upper Peninsula of Michigan. Some possible search sites include the Central, Central Exploration, Cliff, Phoenix, Mandan, and Delaware mines as well as some of the shoreline outcrops near Eagle Harbor.



Mineral of the Month Archives

May 2007: Rainbow Fluorite

June 2007: Lake Superior Michipicoten Agate

July 2007: Labadorite

August 2007: Rain Flower Agate

Fall 2007: Malachite

December 2007: Nepheline Syenite

January 2008: Native Copper

February 2008: Amazonite

March 2008: Lake Superior Agate

April 2008: Shadow Agate

May 2008: Apohpylite

June 2008: Ocean Jasper

Summer 2008: Marra Mamba Tiger's Eye

September 2008: Mohawkite

October 2008: Mexican opal

November 2008: Prehnite

December 2008: Picture Jasper

January 2009: Sea Shell Jasper

February 2009: Polychrome Jasper

March 2009: Selenite Desert Rose

Spring 2009: Coyamito Agate

July 2009: Obsidian Needles

August 2009: Goethite

September 2009: Banded Iron Formation

Fall 2009: Fairburn Agate

March 2010: Fossilized Dinosaur Bone

April/May: 2010 Kentucky Agate

June 2010: Nantan Meteorite

July 2010: Mookaite Jasper

Aug/Sept 2010: Polyhedroid Agate

Fall 2010: Ammonite Fossil

September 2011: Petoskey Stones

Spring 2011: Petrfied Wood

Winter 2011: Argentina Condor Agate

January 2012: Mary Ellen Jasper

March 2012: Mexican Crazy Lace Agate

June 2012: Moqui Marbles

September 2012: Chlorastrolite Greenstone

March 2013: Jacobsville Sandstone

August 2013: Unakite

November 2013: Skip-an-Atom Agate

April 2014: Tiger's Eye

September 2014: Black Corundum

February 2015: Condor Agate

June 2015: Petoskey Stone

November 2015: Slag

June 2016: Lake Superior Copper Replacement Agates

March 2017: Chert

July 2017: Kona Dolomite

December 2017: Septarian Nodule

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Gitche Gumee Museum.
E21739 Brazel Street
Grand Marais, Michigan 49839


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