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June 2015 - Petoskey Stone (Michigan)

Okay, I will admit it. I didn’t realize until now that in this version of my webpage I have not yet featured the state stone for Michigan: the Petoskey stone. As I have changed webmasters over the last dozen years or more, the content of the webpage has also changed since I have started fresh with each new webmaster that had his or her own server. I have been with my current webmaster since May 2007 (thanks Michael!). So don’t you think it is about time that I include my state’s stone as mineral of the month?

A Petoskey stone is both a rock and a fossil. The rock is fossilized remains of a particular species of coral that lived during the Devonian period -- rugose coral, Hexagonaria percarinata.

They are fragments of a coral reef that was originally deposited during the Devonian period (419–359 million years ago). At that time a shallow ocean covered what is now the lower peninsula of Michigan. Diagrams of the Michigan Basin and the rock that made up the geology of this basin are below.

The coral reefs that formed Petoskey stones are shown in the Devonian rock colored red in the two diagrams below. Specifically, it is found in the Gravel Point Formation of the Traverse Group.


When dry, the stone resembles ordinary limestone but when wet or polished the distinctive mottled pattern of the six-sided coral fossils emerges.


Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). Most limestone is composed of skeletal fragments of marine organisms such as the coral that makes up Petoskey stone. Limestone makes up about ten percent of the total volume of all sedimentary rocks in the Earth’s crust.

These state stones can be found because they were plucked from Michigan’s bedrock by glaciers that last retreated from the area around 10,000 years ago. Erosional forces ground off the rough edges of these pebbles and deposited them primarily in the northwestern (and some in the northeastern) portion of Michigan's lower peninsula. In these areas, complete fossilized coral colony heads can be found in the source rocks for the Petoskey stones. In 1965, it was named the state stone of Michigan.



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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