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MINERAL OF THE MONTH

September 2011 - Petoskey Stones

In 1965 the state stone for Michigan was designated as the Petoskey stone. Petoskey stones are fossils of coral colonies called Hexagonaria. "Hex" comes from hexagon, because the coral is usually a six-sided polygon. The coral grew 350 million years ago, 150 million years before the dinosaurs! It grew only in shallow tropical salt-water seas where lower Michigan is now. The coral stacked in layers, then fossilized into rock.

The Petoskey stones found on the beaches of Lakes Michigan and Huron were formed as a result of glaciation, in which sheets of ice plucked stones from the bedrock, grinding off their rough edges and depositing them primarily in the northern portion of Michigan's Lower Peninsula. In some areas of Michigan, complete fossilized coral colony heads can be found. The formations and specimens found inland tend to be rougher since they have not been weathered as much by the wind, water, and sand from the shoreline. The movement of the frozen lake ice acting on the shore during the winters is thought to turn over stones at the shore exposing new Petoskey stones at the water's edge each spring.

Petoskey stone on beach

Similar fossil corals occur in a variety of locations; however the name Petoskey stone should only be applied to those from Michigan which exhibit a six-walled coral structure that has the distinctive "eye" pattern within each cell.

Petoskey stone polished
Petoskey stone close up

When ancient glaciers pressed down on the center of the Lower Peninsula, the layer of fossilized coral rose up to form a ridge and created the dish shaped "Michigan basin". The exposed layer of rock is where Petoskey stones come from. Petoskey stones are found in the Gravel Point Formation of the Traverse Group. They are fragments of a coral reef that was originally deposited during the Devonian period. When dry, the stone resembles ordinary limestone but when wet or polished using lapidary techniques, the distinctive mottled pattern of the six-sided coral fossils emerges.

Petoskey stone - rough
Petoskey stone - rough

It is sometimes made into decorative objects. Other forms of fossilized coral are also found in the same location including the closely related favosites. Favosites is an extinct genus of coral characterized by polygonal closely-packed corallites (giving it the common name "honeycomb coral"). The walls between corallites are pierced by pores which allowed transfer of nutrients between polyps.

Favosite - Rough
Favosite - Rough
Favosite close up

The name comes from an Ottawa Indian Chief, Chief Petosegay. The city of Petoskey, Michigan, is also named after him, and is the center of the area where the stones are found. According to legend, Petosegay was the child of a descendant of French nobleman and fur trader, Antoine Carre and an Ottawa princess. Petosegay, meaning "rising sun", "rays of dawn" or "sunbeams of promise", was named after the rays of sun that fell upon his newborn face. In keeping with his promising name, Petosegay was a wealthy fur trader who gained much land and acclaim for himself and his tribe. He was remarked upon to have a striking and appealing appearance, and spoke English very well. He married another Ottawa, and together they had two daughters and eight sons. In the summer of 1873, a few years before the chief's passing, a city began on his land along Little Traverse Bay. The settlers christened the newborn city Petoskey, an anglicized form of Petosegay.

 


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

June 2012: Moqui Marbles

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

Winter 2011: Argentina Condor Agate

Spring 2011: Petrfied Wood

September 2011: Petoskey Stones

January 2012: Mary Ellen Jasper

March 2012: Mexican Crazy Lace Agate

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

August 2016: Puddingstone


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