If you’re from Michigan, or have ever visited Michigan, you probably know all about petoskey stone. After all, it is the “state stone” of Michigan.
But I’m not from Michigan, and so my husband had to educate me; allow me to share the story with you.
When taking a stroll along a Lake Michigan beach in northwestern Michigan (on the lower peninsula), keep your eye out for promising pieces of limestone. Pick a stone and wet it a little—if you’re lucky, a web of crooked and variously-sized hexagons will appear in the stone right before your eyes.
It can be nearly impossible to tell regular limestone apart from a petoskey stone unless the stone is wet. In fact, collectors often carry a small container of water with them as they look for the stones so they can tell on the spot if they’ve found what they were looking for. (Other collectors prefer to go out in a light rain so they can more easily find these special stones.)
When a petoskey stone is dry, it can be hard to tell it apart from ordinary limestone.
The stones get their name from the resort town of Petoskey, Michigan, where they were (and still are) sold to tourists as souvenirs.
Petoskey stone is technically limestone, but can you guess how the distinctive hexagon shapes came to be? If you guessed that they are fossilized coral, then you are right. (The stone in the first photo is a little unusual in that you can see the ridges of the sides of the coral running down the sides of the rock, in addition to the distinctive coral shape at the top that is the hallmark of a petoskey stone.)
Coral, of course, favors warm salt water, and Lake Michigan is a fresh water lake that isn’t particularly warm most of the time. So how did fossilized coral end up in Michigan? Well, scientists believe that a very, very long time ago, the land now known as Michigan was near the equator. Not only was it warmer, but it was covered by a huge shallow sea.
Over time, the land mass broke apart and our modern continents began to take their shape. The land under the shallow sea was lifted up a bit and the shallow sea dried up. As the land mass of modern Michigan shifted northward, the climate also became too cold to support tropical sea life such as a coral reef.
You may then wonder how the coral became embedded in rock. The coral was likely buried in mud filled with lime (calcium carbonate) that was covered by more and more mud over time. As the water was squeezed out, the intense pressure hardened the mud and coral into stone.
Petoskey stones can be highly prized and are often polished to bring out the beauty of the coral.
Here’s a link that gives the history of petoskey stones and provides guidance for hand polishing any treasures you may find on the shores of Lake Michigan.
Be sure and share your rock hunting stories with us here at TMG, and photos of special rocks you find on the TMG flickr pool.
Wishing you happy rock hunting,