Kathy Miling of Cadillac, MI was a previous Shop Friend of the Month with her entry Polishing Petoskey Stone. Lapidary techniques are varied but give everyone a starting point and each person will develop their own system as they go. Kathy wrote the following: "I have polished hundreds of Petoskey stones... leaving many of them the shape they are and some of the larger ones I slab and make cabs out of them. They are very interesting when polished. If you put a Petoskey stone in a vibrating tumbler (Lot-O-Tumbler) with 220 grit for more than 12 hours, you will have mud. They are very soft, so you have to add more water than usual (like 1/2 capful) and start checking after 8 hours. They can't be tumbled in a rotary tumbler. The size of the Petoskey stones I put in a tumbler are from 2" and smaller. Be sure to mix the large and small sizes in one barrel. I only use a vibrating tumbler to take off the outside roughness and I watch it really close because they will disappear before you know it. So then I finish them by hand, using silicon carbide sanding belts on an expandable drum wheel with 200 grit, 400 grit, then 600 grit covering every inch of the stone. If they are too large to go into a tumbler, I start out on a wheel of 100 grit, but press very gently, then continue as mentioned above. Then I soak them in mineral oil at about 200 degrees for 2 hours. When you take them out of the oil, wipe them down and then finish them on a buffing wheel. First, use a dark brown buffing compound, E5 Emery by Dico, found at http://www.dicoproducts.com/guide.html
using a short buffing cloth (Cotton Flannel Buffs that are stiched with hardened centers, (get these from http://www.alpha-supply.com) and then the blue compound PBC Plastic by Dico, with a longer buffing cloth, (Regular Muslin Wheel Buffs). I usually put two of them together. (You can get this also from Alpha Supply.) The buffing cloths are attached to an arbor. I have a polishing motor... it has a rod coming out of the motor and then you can put the buffs over the rod. Get the motor going, then rub the buffing compound onto the buffs and then just polish the stone, holding it up to the buffs... continue to add the compound as needed. Wear a mask!
I would also like to add that the Petoskey Stone is composed of a fossilized skeleton of coral which lived in the salt water that covered Michigan during the Devonia Period, about 350 million years ago. The hegagonal chamber is the prehistoric remains of the marine life. They are now found along the northern shores of Lake Michigan and in gravel pits of the northwest side of Michigan." See Kathy's new web site: http://www.getpetoskeystones.com
Kathy, thank you very much for your great detailed article! Your polished Petoskey looks fabulous!