The Petoskey Stone 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 vibration 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 in 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. Be sure to mix the size of the rocks. After 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 using 200 girt, then 400 girt, and then 600 grit with water and working every inch of the stone with each step using a circlular motion, pressing gently and always moving. Be sure to wash between each step. 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 while grinding. You want to always keep the wheel wet while working the stone. Then I soak them in mineral oil at 200 degrees for around2 hours. Wipe off excell oil and then finsih them on a buffing wheel. First use the dark brwon buffing compound E5 Emery by Dico using a short buffing cloth (cotton flannel buffs) that are stiched with hardened centers. Then use the Blue compound PBC Plastic by using a muslin wheel buffs. I usually put two of them togeather. The buffing cloths are attached to an arbor. Rub the buffing compound 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 petoskey stones that are too large to go into the tumble, I start out using a 100 grit or even 80 grit for the really pitted ones.
Good Luck and enjoy the beauty of the Petoskey Stone.
To learn more about Petoskey Stones visit my website at http://www.naturestreasures@officeliveusers.com
