How to Polish Rocks
Have you ever walked along a beach or a riverbank, and found a rock you just had to have because it was so smooth and pretty? Over many years water, wind, smaller rocks, and sand have ground against that stone (and others) to achieve that look and feel. It could have taken a hundred years to get the rock to that point! Luckily for us, you can do it yourself at home in just a few weeks using a rock tumbler, and a little bit of patience.
Now, there are two kinds of tumblers; a rotary (like above) and a vibratory type. This guide is geared towards the rotary tumbler, but many of the steps also apply to vibratory tumblers. The biggest difference being that vibratory tumblers are quite a bit faster, and use less grit, but have a bigger learning curve and are not used to shape rough rocks, only to polish.
Essentially a rock tumbler is a barrel (typically rubber), filled approximately 2/3 - 3/4 full with rocks, sand (called "grit"), and water. After being filled, the barrel is shut with a screw on lid, and placed on an electric chassis that rotates ("tumbles") or vibrates (vibratory type) the rocks around, 24 hours a day. Think of it like a "cheat sheet" for nature!
As for what to polish, the sky is the limit! You can buy rocks here, you can find rocks in your yard, or trade with local rock hounds in your area. Rocks come in different hardness levels and are based on the Moh's scale, a scientific tool designed to tell us roughly how hard a specific rock is. A good general guideline is to mix rocks within 1/2 step hardness of each other. Harder rocks tend to polish better but they also take more time. Patience is key with tumbling.
Try to choose rocks that are very hard, with no cracks or pits. These imperfections can go quite deep which costs you more time & grit to get good results. These rocks will also require more cleaning as you go through the stages, and sometimes a small pit can become a much bigger one as the grit erodes the surface. Garbage in means garbage out. If you MUST tumble a stone like this, run it through stage one until it's mostly smoothed away.
Next, fill your barrel up! Keep in mind that you don't want it too full. If you overfill it, you won't get a lot of grinding going on and you will cause undue stress on your machine. Remember, a 4 pound barrel means "barrel + rocks + water + grit = 4 pounds" (ish). There is some leeway here but use common sense. If your rollers are struggling to move, lighten the load by removing some rocks.
On the flip side, if you don't fill it full enough, you will get the rocks slamming into each other. You want enough space for the rocks to move against each other gently, like people in a crowded room. You don't want a mosh pit situation where they're banging into each other and going crazy -- this leads to bruising and cracks. You can use some ceramic media to fill in the extra space if you don't have enough rocks.
After loading up your rocks, add your water. Enough water that you can see it almost to the tops of the rocks or just over. It's OK if it's not exact. Lastly, add your grit. Rough rule is 2 tablespoons of grit for pound of rock (depending on stage). When grinding rocks that are very jagged and rough, we might add a little bit more, but would use less grit when starting with rocks that are almost totally rounded. If you look in the barrel at the end of a week, and you can still see visible grit particles, you've added too much. Try less on your next fill.
Now that you've got your rocks sorted, and your barrel loaded, it's time to set it and forget it (for a few days, anyways). Rock tumblers are not terribly loud, but it's a good idea to set your machine up in a laundry room, a closet, or in garage. Pick a spot out of the way where it can run 24/7 without interruptions or obstruction. Remember that rubber barrel rock tumblers are much quieter (and durable) than their plastic counterparts.
Initially you're going to check the barrel once or twice a week to see to see how things are going. After each batch of grit is run, the rocks should be checked, and the barrel refilled with the same rocks, water (the same, or fresh) and fresh grit. You do not need to clean the barrel out 100% if you are continuing the same stage, just add fresh grit. There's nothing wrong with doing a full clean out every week either, I just find it to be more mess than it's worth.
You may also want to burnish your rocks between stages, to help ensure all the excess grit and grime is washed away -- more on this below.
Whichever method you choose, make sure you get all the grit and rock particles from the previous stage cleaned off before you move to the next stage. You do not want a piece of grit or rock from a previous stage coming into the new stage, or it can undo all your hard work. Also, never be afraid to go back and re-run a stage. It's all about what your desired results are. There is no wrong method!
Rinse your rocks using a colander and a 5 gallon bucket. DO NOT dump your grit slurry down the drain. Rock tumbler slurry is called "Plumbers Best Friend" for a reason. Your slurry can be buried in a corner of your yard, or dumped on a rocky patch. Keep it away from pets. If the weather is nice, dump it in a cardboard box in the sun. The water can seep out, and the grit will stay contained for easy disposal once it's hardened up into a puck.
Typically stage 1 is 10-21 days; entirely up to you and your desired results. If your rocks are already very round and smooth, this stage will be short (or may not be needed at all). Any rocks that are pitted or cracked after stage 1 can be discarded, or run again in this stage until you're happy with their appearance. Check every couple days. Rocks should be pretty smooth after stage 1.
Stage 2 - medium grit - approximately 7-10 day. Use 2 tablespoons of grit per pound of rock. If you have lost a lot of mass due to grinding in stage 1, this would be the time to use ceramic media to fill in the empty space.
Stage 3 - fine grit and/or "pre-polish" - is approximately 7-10 days. Use approximately 2.5 tablespoons of grit/pre-polish per pound of rock. This stage can be done with a "fine" grit, or with an aluminum oxide "pre-polish". You can even do both, just run the fine grit first for a week, then the pre-polish for another week. It will only make your final results better!
Once your rocks are done this stage, rinse them well and dry them completely. Remember, rocks will appear much shinier when they're wet. Buff them with a soft cloth and they should have a slight sheen to them. If no sheen, put them back for another run, or try burnishing them.
This will get everything squeaky clean, and ready for the final polish! You're on the home stretch!
Burnishing is a fancy word for "deep cleaning". Basically burnishing means you give your barrel a quick rinse so no grit remains, then put the rocks, fresh water, and soap (Ivory, Borax or similar) back into the barrel. Let it run for a couple of hours. This gets rid of any grit, or residue that could cause you issues.
After all of that, time for the final polish! This is what we've all been waiting for.
Make sure your barrel is totally clean, and fill it 3/4 full with rocks and/or top up with media if you need. Add 3 tablespoons of polish per pound of rock. Add a 1" cube of soap (or a tablespoon of shredded soap, or powdered Borax) to help provide a wetting action, and get everything nice and shined up. Run this mix for about 5-7 days; checking in on the 5th day. Remember, rinse & dry the rock fully to assess it. if you're happy with the results, time to do a final clean.
For a final cleaning, just repeat the burnishing steps -- rinsed rocks, soap and water in the barrel. Running this final burnish for up to 24 hours is a good rule of thumb. Just don't go too long or you could wear off your polish.
You're done!!! You have officially tumbled your first load of rocks! Now you have a batch of rocks for display, for making jewelry, art, magnets, or simply just vase filler. Feel free to tag and share them with us on Instagram. We'd love to see what you've accomplished!! Remember, the camera will pick up every imperfection (this is both good and bad) :-)
Full disclaimer: The polished rock picture in this post is from one of my earliest batches of rocks. Notice how you can still see some pits if you look really close? That's called "being too impatient" and "the camera doesn't lie" :-) The worst of these got re-run in a later batch. No shame!