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How to Polish Your Michigan Petoskey Stone

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Tracy Stengel
Tracy Stengel

The beloved Petoskey stone. Nothing gets old about this 350-million-year-old fossil.

From Spring to Fall, millions of people come from all over the world to comb the shores of Northern Michigan in hopes of finding a Petoskey Stone. Memories are made during the hunt. Blue, cool, water licking ankles, toes touching sand, a breeze in your face … and then you see it … a rock looking back at you. Children and adults squeal with delight. There are high-fives all around.

But then, you get it home, intending to display your prized find and blink twice. Now it’s dry. What happened? Where did the eyes go? Is it sleeping or did it morph into a plain grey rock on the trip home? It looks nothing like the beauty you found in the water and far from the shiny, glass-like stunners you saw in the gift shops.

As an avid rockhound, I want to share my step-by-step guide to polish your treasures. It is a simple method, requiring sandpaper, time, and a lot of elbow grease. The reason Petoskey stones can be polished by hand is because they are relatively soft. On the Mohs scale of mineral hardness, from 1–10 a Petoskey scores a 3. To put it in perspective, talc scores a 1 and diamonds score a 10. This method won’t work with many other wonderful Michigan stones such as an agate which scores an 8.

It is important to remember to always work with a wet stone. You don’t want to breathe in the dust created by sanding a dry stone. It can damage your lungs.

If you are starting with a pitted or uneven Petoskey, use a metal file to shape and smooth out the rough patches.

I do most of my polishing in the garage because it can get a bit messy. I also take off my rings, so I don’t scratch them. I set a small bucket with a few inches of water at my feet and sit on an outdoor chair with a towel on my lap to protect my clothes.

For beginners, I recommend starting with a relatively smooth stone that is “pure Petoskey,” meaning when you wet the stone, you can see the pattern clearly throughout.

Begin with 220-grit silicon carbide waterproof sandpaper found in most hardware stores. Hold the rock firmly in one hand and start sanding with the other. Go in a circular motion. If a back-and-forth motion is easier for you, be sure to keep turning the stone to avoid creating any grooves. Use your fingers to detect rough spots that may not be visible. Once the stone feels smooth, let it dry. Inspect it for any scratches or pits. If you detect any, wet the stone and work on those areas with the 220-grit paper until the stone is free of blemishes when dry. This step will take the longest and it the most important.

Repeat the process with 400 and 600-grit silicon carbide waterproof sandpaper. You can go up incrementally from 800 to 3000-grit paper, but I don’t think it is necessary. If you are thorough with each step, you can stop after 600-grit and still get that “gift shop gloss.”

Make sure to change your water and rinse your stone often. Between each step, remember to dry and inspect your stone. If there is a flaw, do not go to the next grit. Whatever blemish you see on a dry stone that was sanded with 220-grit paper isn’t going to disappear when you move to 400-grit. Save yourself the time and frustration and be patient with each step. Once you are satisfied with how your dry stone looks after sanding with the highest grit level you are using, at least 600, now it’s time to polish!

I use Zam, a jewelry polish you can find online. Some folks use a car-finish rubbing compound, but that hasn’t given me the high shine I want. Zam comes in a tube and resembles a very hard, green wax. I chip a few flakes off the top of the tube with the end of a screwdriver or knife and let them fall on a piece of corduroy or velvet. Use a rotating rubbing motion to work the polish compound onto the stone. Wipe with a clean, dry cloth. You should have a beautiful, perfectly polished Petoskey stone you can cherish forever.

If you aren’t happy with your stone after polishing, go back at least to the 400 grit and repeat the process.

Polishing Petoskey stones requires patience. Remember: nothing good ever comes easy! This pure Michigan pastime, while sometimes tedious, is tremendously rewarding.

Show me your stones in the comments or share your polishing tips.

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