MINERAL OF THE MONTH: 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.



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