I have received several calls from Customers who have gathered Petoskey Stones (the “State Stone” of Michigan) from beaches in Northern Michigan near the towns of Petoskey and Charlevoix, who wanted my “recipe” for tumble-polishing them. These stones are really a gray-colored fossilized 6-sided coral from ancient coral reefs, and are very soft (Mohs 5). Most internet articles advise collectors to hand-sand and hand-polish these stones, because they are so soft.
I had a strong feeling that Petoskey Stones could be tumbled successfully, but since I had never had the opportunity to tumble them, I went to E-Bay and purchased a large coffee can of Petoskey Stones for an experiment.
Sizing Petosky Stones
I sized them to 1-quarter X 2 quarters and rinsed them thoroughly. I put them in a barrel with 25% Ceramic Shapes and fired up my rotary tumbler for Stage 1 (80-grit SiC). I checked them at the 2-day point to see how quickly they were shaping up, then let them go another day and a half (3 ½ days total). I then ran them in Stage 2 (220-grit) for 2 ½ days, in Stage 3 (600-grit) for 2 days and in Stage 4 (1000-grit) for two days. The stones looked perfect for polishing, so I polished them for 5 days in Cerium Oxide and was shocked to see that they had undercut badly and looked worse than they did after Stage 4.
The internet sources recommend that you stop tumbling after Stage 2 and simply hand sand them with wet/dry sandpaper (or) run them on a 600-grit Silicon Carbide or diamond flex-wheel.
The internet sources further advise that you soak the stones in mineral oil for 2 hours at 200oF in your oven--I thought that method was too hazardous and might burn down my home, so I put the stones and the mineral oil in a double boiler with water in the lower container. I then carefully heated the water in the double boiler on the stove top until the oil reached 200oF, removed it from the heat and let it sit for 60-minutes--I reheated the mineral oil to 200oF and again removed the heat & let the stones cool in the oil for another hour.
Finishing Petosky Stones
When the stones had cooled, I put them on paper towels to draw off the excess mineral oil. Then it was time to polish the little troublemakers by hand! You can use either a Polish Stick, a hand buff, a cotton buffing wheel, a flat lap or a vertical polishing wheel to polish the stones. The internet recommends two stick polishes--first with Dico Products Emery (P/N 531-E5) and second with Dico PBC-Blue (P/N 531-PBC)--these can be found at Ace Hardware Stores. I prefer to use a vertical polishing wheel and used Cerium Oxide with great success. The key to polishing these little guys is the 600-grit sandpaper, which eliminates the undercutting (undercutting is the erosion of some layers, but not others, thus leaving a rough surface).
Petoskey Stones are a strange breed. The undercutting issue prevents normal mass-polishing techniques, and so must be hand polished one at a time. They are quite beautiful when you finish them by hand, but I have to admit I was really surprised that they got far worse in the polish slurry inside the tumbler. I would have bet a lot of money that they could be tumble polished--I’m glad I didn’t lay a bet down!
If you would like more information about key procedures for rock tumbling (but not Petoskey Stones!), we advise that you pick up a copy of our book, Modern Rock Tumbling, which will make you the neighborhood expert on how to do it and why!