Michigan’s Official State Stone The Petosky Stone

(Hexagonaria Pericarnata; Sloss)

Petosky stone

     The Petosky Stones were once living creatures called colony coral, about 350 million years ago.  This time was called the Devonian Period,  which lasted 50 million years,  in the middle of the Paleozoic Era.  The Petosky-stone corals lived in the warm salt-water that covered Michigan at that time.  Other salt-water marine life can be found as fossils along with the Petosky Stones, such as brachiopods, solitary and horn corals, trilobites, cephalopods, gastropods, bryozoans, and crinoids. Some Petosky-stones can grow up to a yard across and 2 feet high.  The Devonian Era is also commonly called  “The Age of Fishes”.  If you are very lucky you can find fish scales or bones,  while you are hunting for Petosky- stones in some of the area’s  quarries. 

     The hexagonal chamber, usually referred to as the “Eye”,  was the mouth and food canal for the animal.  The radiating lines from the mouth,  always seen on the surface of the Petosky Stones, were the attachment areas for it’s tentacles.  The animal’s tentacles drew microscopic particles of food into the coral’s mouth.  Any flecks you see on the Petosky-stone’s surface, may have been some food particles which were fossilized right along with the coral.  The darker areas and streaks are filled with silt, which was buried, with the coral.  The actual coral is made out of calcite in the form of Limestone.  Many collectors like the dark spots with lighter outer edges.

     One method of hand polishing a Petosky-stone is very simple.  Find a Petosky-stone that has a pattern on the surface that you like.  You can use a hack-saw to cut the stone down to the size you need.  Then, you can use a file, grinding wheel, or a sharpening stone like a whetstone, to shape the Petosky-stone to the desired shape.  Once you have the basic shape finished,  get a container with clean water in it.  Use a rough (120 to 180-grit) wet or dry sandpaper to sand and further shape the Petosky Stone, keeping the stone and the sandpaper wet at all times.  Wash the whitish residue off in the clean water can frequently.  Once the Petosky-stone is as smooth as possible, switch to (240-grit) sandpaper.  Wash the stone and your hands thoroughly between each grit.  Repeat with (360 and then 600-grit) till the Petosky-stone is as smooth as possible.  To polish, use a piece of flannel, velvet, carpeting, or felt with tin or cerium oxide polish and water on it.  Some toothpastes can also make a good polish.  Rub stone till the polish is the best you can get.