If you want to enjoy and share all the intricate details of your Petoskey Stones, you’ll need to polish them to a luminosity that lives up to their name, accenting the radiating lines surrounding each corallite eye and giving the stones a 3D appearance.
There are several polishing methods that can be used to meet these ends. It’s important to note that Petoskey stones are fairly soft. On the Mohs hardness scale, which ranges from 1 for talc to 10 for diamonds, Petoskey stones rate about a 3. This makes them fairly easy to sand by hand, and a little risky in a tumbler or at the mercy of other power tools. Whatever method you use, patience is key to a high-quality result.
How to Polish Petoskey Stones by Hand
Petoskey stones consist largely of calcite and silica, which have replaced the original living cells of the organism. The stones respond well to hand sanding. Your goal is a well-polished stone without scratches or edges.
You will need:
- A Petoskey stone
- Sandpaper of various grits, from 100-800, depending on how rough your original stone is
- A towel or some old newspring
- Soft fabric such as felt or corduroy, or denim, or you could use leather
- Polishing powder, such as TXP aluminum oxide
1. Choose and wash your stone.
Smoother stones collected along the water’s edge are a better choice for hand polishing. Expect to spend several hours in order to achieve your desired shine. Begin by thoroughly cleaning your stone of choice. An old toothbrush can help dislodge dirt or debris clinging to the stone.
2. Sand away all scratches.
If you are sanding a lakeside stone, 220 grit sandpaper is a good place to start. You may rub the stone on the sandpaper, or the sandpaper on the stone, whichever seems easier for you. After thoroughly sanding the entire surface, rinse and dry it for inspection. If there are still scratches or other marks, continue sanding and checking until the stone is completely free of defects.
Continue sanding with a finer grit, such as 400, to remove any coarse areas. Remember to periodically rinse, dry, and inspect the stone. Repeat with 600 or 800 grit, as needed. When you feel the stone is just about perfect, keep going for another 10 minutes or so just to be sure.
3. Polishing your stone.
Polish your stone to a high gloss by sprinkling polishing powder on a soft fabric and working it in with short, rotating movements. If you notice new scratches showing up on the stone, go back to the sanding stage until they’ve been removed.
After applying the polish, rinse the stone with clean water and dry with a soft cloth.
How to Polish Petoskey Stones With a Dremel Tool.
Petoskey stones can be polished with a Dremel tool. The process is similar to hand sanding but is easier on the hands and fingers over extended periods of time. It also is easier to use on slightly rougher stones. You will need sanding pads from 50-1500 grit along with a buffing/polishing wheel for achieving the desired finish.
If you are starting with a rougher stone, you will need to make sure its surface is free of dirt that may cause unwanted scratches. Again, use an old toothbrush to clean all crevasses and rough places in the stone. Rinse thoroughly and dry. Then place the stone on a hard, flat, and stable surface.
Begin with a 50-grit sanding drum pad, working over the entire surface of the stone.
Continue to gradually change drum pads until you’ve reached the 1500-grit pad and the stone is as smooth and polished as you want it to be. You may want to rinse and check the stone periodically to make sure no blemishes remain.
Change to the buffing/polishing wheel. add the polishing powder to the wheel and continue to polish to a high shine. If the stone begins to seem oily, add denatured alcohol to the wheel and continue polishing until the oily look is gone.
How to Polish Petoskey Stones With a Rock Tumbler.
Because of the softness of Petoskey stones, trying to polish them in a regular rock tumbler can be a tricky process. Many online sites warn against trying it. However, if you have really rough stones, it can be a good way to remove sharp edges and jagged surfaces. You will need to monitor the process closely. You may also want to stop short of the polishing stage and continue by hand.
Some websites recommend using a vibrating tumbler rather than a rotating one for these softer stones. But even so, leaving your stones for too long in a rough-grit medium can result in no more than a tumbler of mud to show for your efforts.
1. The first grinding stage of tumbling.
Begin with just a few stones, around 2 inches or less in size. Surround the stones with ceramic media to help cushion them while tumbling. Use a ratio of 50/50 stones and ceramic. Add 1 tablespoon medium (180 or 220) grit for each pound of total stone plus ceramic media in the barrel.
Add enough water to cover the contents of the tumbler. Some sites recommend thickening the medium with a bit of honey or molasses to add even more cushioning, but don’t overdo it.
Seal the barrel and tumble for one or two days, then open it and check the progress of the stones. The edges and sharp points should already be turning smooth. If the stones seem smooth enough, wash them for the next step. If not, continue tumbling for another day, then check again. Continue to check daily until the stones are smooth. It shouldn’t take more than 3-4 days at most.
2. Smoothing the stones.
At this point, an inspection of the clean stones may reveal breaks or flaws in some that will prevent them from polishing up as you would desire. It is probably best to discard these stones, as continued tumbling will only damage them further.
Thoroughly clean the tumbler barrel, and start the process again with a fine (500 or 600) grit abrasive medium. After one or two days, change again to an ultra-fine (1000 grit) medium. One day in the ultra-fine grit should be enough to make the stones ready for polishing. Clean the stones and the barrel again thoroughly.
3. Ready for polish.
Here is where many recommend you forgo further tumbling in favor of hand polishing your Petoskey stones, or change over to a vibratory tumbler to protect the stones from damage. Hand polishing may be done as described above with soft fabric and a quality rock polish.
If you decide to use a vibratory tumbler, fill the clean barrel with corn cob media mixed with one tablespoon of TXP polish per two-pound of bowl capacity. Add just a few stones to lessen the chances of them touching each other. Tumble for 24 hours and check to see how the stones are accepting the polish. If the media is caking on the stones, there are too many in the barrel. Try removing half the stones, and then continue polishing.
One or two days should be sufficient to polish stones, but you can continue longer until there is no further improvement in the quality of the shine. Going any longer may actually cause the shine to start disappearing. It’s no small accomplishment to achieve a nice shine by using a tumbler, so enjoy your bragging rights.
How to Polish Petoskey Stones With Lapidary Tools.
Experienced rockhounds may also polish Petoskey stone with more professional lapidary tools. The belted tools can grind down the soft surface very quickly, making the whole process much quicker. An average stone can be polished in around 30 minutes with lapidary equipment. However, caution will need to be used in order to protect the delicate surface from damage.
A stone’s rough surface can be shaped as it is wet-sanded on a rubber wheel with a 220-grit silicon carbide belt. A silicon carbide grinding wheel can also be used. Take it slow and add lots of water to cut down on dust.
Once the stone is smooth and well-shaped, change to a 600-grit sanding belt, then 1000-grit if available. Hand polish for the ultimate finish. Although lapidary equipment is quicker, patience and careful attention can produce a beautifully polished Petoskey stone that’s worthy of any rockhound’s collection.