What is a Petoskey Stone
The Petoskey Stone is the Michigan's State stone and is beautiful when polished.
The Petoskey Stone is a fossilized coral that lived 350 million years ago during the Devonian age. The scientific name is hexagonaria Percarinata (six sided).
The Petoskey Stone was a living flower animal and each hexagon was the tip of a single tentacle, which was where the food was absorbed. The tentacles were all pushed togeather and the plant petrified over millions of years. The coral was orignally red in color, sometimes you can still see the red color .
The PS is only found in north west Michigan along the shores and inland of Lake Michigan.
The photo on the right and below shows the Petoskey Stone unpolished but tumbled for a few hours in 200 grit.
How to Polish a Petoskey Stone
I have polished hundreds of PS...leaving many of them the shape they are and sold as paper weights or for the collector and some of the larger ones I have slabbed and made cabochons for jewelry.
Many Petoskey stones are not petoskey all the way through...so,
then I would just polish the face side and leave the back side rough.
Or you can polish one side even when the whole stone is petoskey...some collectors like to have one side show what it looks like in the rough.
The Petoskey Stone can't be tumbled to full polish...You have to polish them on a
expandable drum wheel and then finish on a buffing wheel.
The PS is very soft, a hardness of 5, so you can't
tumble it to a full polish. The first outer roughness can be taken off with the
vibrating tumbler. I use 220 grit and just enough water to wet the rocks and tumble for between 8 to 12 hours, watching it closely and adding water as needed. You want the rock to look like it is covered with wet mud as it is rolling in the tumbler, don't let the mud dry.If you leave them in there too long the rock can eventually disappear. They can't be tumbled in a rotatary tumbler because it will bruse them. The size of the PS I put in the tumbler are around 2" and smaller. Be sure to mix some large and small ones togeather. After the tumbling has removed the outer roughness I wash them off and work them by hand using silicon carbide sanding belts on an
expandable drum wheel with 200 grit, then 400 grit, and then 600 grit with water and covering every inch of the stone with each step using a circlular motion, pressing gently and always moving. I use those fish tank bubblers to throw water onto my wheel while grinding and shaping. Some machines are equiped with a water drip that drips water on top of the wheel. Always wash the stone inbetween each step and cleaning the water after each step. Then I soak them in mineral oil at 200 degrees for aound 2 hours. Whip off the excess oil and then finish them on a
buffing wheel. First use the dark brown buffing compound (E5 Emery by Dico found at
http://dicoproducts.com using a short buffing cloth (cotton flannel buffs) that are stiched with hardened centers. (get these at
http://www.alpha-supply.com)and then use the Blue compound PBC Plastic by using a muslin wheel buffs. I usually put two of them togeather. (you can get these also at Alpha Supply). The buffing cloths are attached to an arbor. Rub the buffing compounds onto the wheel first and then hold the stone up to the cloth with circular motions, covering the whole stone. Continue to add the compound as needed until you get a beautiful final finish.
For the PS that are too large to go into the tumbler, I start out using a 100 grit or even a 80 grit for the really rough ones, then continue as mentioned above.
Good Luck and enjoy the beauty of the Petoskey Stone !!!