How To Polish Petoskey Stones with a Tumbler
Petoskey Stones are fossilized remains of a colonial coral (Hexagonaria percarinata) that lived in the warm Michigan seas during the Devonian Era (about 350 million years ago). Calcite, silica, and other minerals have replaced the living tissues of the original organisms. They are most commonly found in and around Petoskey, Michigan, the city after which the fossil is named. Metaphysically, Petoskey Stones encourage psychic awareness and provide a feeling of protection.
Petoskey Stones are relatively soft, with a Mohs hardness of 3 to 3-1/2, so tumble polishing them safely can be a little tricky, and is recommended for experienced lapidarists. Don't mix them with any rock or mineral harder than 4 Mohs! These instructions are meant to supplement the instructions your received with your tumbler, or our Rock Tumbling Instructions.
Step 1: Start with 320 or 500 & finer silicon carbide grit, using a thickening or suspending agent like syrup, sugar, or molasses for cushioning.
Step 2: Use 600 or 800 & finer silicon carbide grit with a ratio of 1 to 1 or 2 to 1 of media to Petoskey stones. Use plastic pellets or coarse-ground walnut shell for media. You may also need to use a thickening agent like syrup, molasses, or sugar (being careful not to thicken load excessively).
If you use 600 silicon carbide grit, follow with 600 aluminum oxide as a pre-polish, jumping to the final polishing stage.
Final Step: Use cerium oxide or 0.5 to 0.8 micron aluminum oxide polish, cushioning with fine-ground walnut shell and a thickening agent like syrup, molasses, or sugar, but do not thicken load excessively.