How to Tumble Rocks

How to Tumble Rocks

Posted by Oak Rocks on 23rd Apr 2024

What is Rock Tumbling?

Rock tumbling is a fun way of polishing multiple semiprecious gemstones simultaneously in a rock tumbler. A rock tumbler is a piece of equipment that has a plastic or rubber-lined barrel that you fill with rocks, water, abrasive grit and sometimes filler. The barrel is then placed upon slowly rotating rails causing the barrel to rotate and causing the abrasive particles to rub against the rocks, so that over time the rocks become rounded, smooth, and shiny. The optimal speed of rotation depends on the size of the tumbler barrel and materials involved.

Typically, a full tumble polish from rough rock to polished takes about 3–5 weeks, and it usually is done in 4 steps. Beginning with a coarse grit, and then a medium grit, then a fine grit and culminating in a polishing step. It is very important to avoid contamination between steps. If coarse grit gets into your medium grit step, it will scratch up the rocks and you might need to do the medium grit step all over again. Avoiding contamination is easy. Just make sure you thoroughly clean the rocks, the tumbler barrel, and your tools when you change from one grit size to another. We will discuss how during the steps.

Materials Needed for Rock Tumbling

Rock Tumbler- there are lots of rock tumblers on the market from cheap ones to high quality. You may want to consider starting with a cheap one, if you are just experimenting-but remember your results may be reflective of the quality of your equipment and materials!

A Rock Tumbling Kit – Rock tumbling kits are readily available and usually have everything you need for your first experience.

Ceramic Media Filler or Plastic Pellets- A combination of large and small pellets works best.

Rocks - There are many rocks available, some tumble well and some don’t. You may want to start with cheaper rocks and move up to better quality. But again, your results will directly reflect the quality of the rough rock you choose! Some kits do come with starter rocks or you can choose your own. The rocks you choose should be between the hardness of 5-7 on the Mohs scale. All of the rocks you put in the barrel should be the same hardness, as harder rocks will destroy softer rocks. Chalcedonies are excellent for tumbling- agates, jaspers, cherts, carnelian, petrified wood, etc. Quartz works too -milky or clear quartz, rose quartz, aventurine, amethyst, smoky quartz, tiger-eye, etc. Obsidian, in its many varieties, is a good choice too. The rocks you choose should be different sizes, but never smaller than 3/8 of an inch or larger than 2 inches. If using a smaller tumbler, keep the maximum size under 1 1/2 inches. Do not use rocks that are porous, crumbly or fractured, really odd shaped or just plain ugly.

Small Strainer/Mesh Bag/Plastic Colander/Nail Brush/Dish Soap- These may be included in kits and are options you may want to consider for checking and cleaning your rocks and equipment

Bucket- To collect and dispose of the muddy grit.

Paper Towels- for wiping and drying equipment

The 4 Steps for Tumbling your Rocks

STEP 1- Coarse Grit Grind

Before you load the tumbler barrel, be sure that it is perfectly clean. There should be no grit, polish or rock fragments left in the barrel from a previous tumble. To prevent leaks, the rim of the barrel and the lid should be totally free from grit or rock particles and should be wiped dry with a paper towel to avoid leaks.

Fill the barrel about 1/2 to 2/3 full of rocks. It is important not to underfill or overfill the barrel so the rocks can move freely, but not get banged around too much.

Add two level tablespoons of 60/90 coarse grit silicon carbide for each pound of rock. Then, add water until the water line is just below the top of the rocks. Seal the barrel, place on the rollers. Though you will probably want to run it out of site because of the noise, you should check it after the first couple hours to make sure there are no leaks. Let the tumbler run for about seven days.

When you open the barrel, the water will be very muddy. You can scoop out a small amount of rocks in a strainer and inspect them. If the rocks still appear really rough or the mud still feels very gritty you may want to run it a few more days. If the rocks look good, dump the contents into a screen, a mesh bag or a colander over a plastic bucket and make sure to immediately rinse off every speck of grit and mud. You can use the small strainer or mesh bag to rinse the rocks in a soapy bucket of water. Do not allow the mud and grit to dry on the rocks and do not dump the mud and grit down your drain!

For best results, inspect the rocks closely to determine if they are ready to move on to STEP 2, or if another week in STEP 1 would improve their appearance. Sometimes tumbling a second week in coarse grit greatly improves their shape and removes more blemishes. Tumbling is a slow process and shouldn’t be rushed! You can set aside rocks that you think are ready to move on and redo any that you believe would benefit from a repeat of step 1. However, if you repeat only some of the rocks, make sure to add new rocks or filler to bring the barrel up to the 1/2 to 2/3 full status.

STEP 2 - Medium Grit Grind

Once all of your rocks look ready to move on to a medium grit, it is extremely important to clean all of the coarse grit and rock mud from the rocks, from the tumbler barrel, and from the barrel lid. You do not want any coarse grit in your medium grit step!

As the rocks are ground, they will be reduced in size. When you return them to the barrel for STEP 2, they will probably not fill the barrel to the recommended 1/2 to 2/3 full level. It is important to have the barrel at least 1/2 full to avoid bruising on your rocks so now may be the time to add enough ceramic media or plastic pellets to bring the barrel up to the recommended level. This is more important for materials that do bruise easy, such as quartz, and less important with varieties of chalcedony. However, if your tumbler barrel travels at more than about 60 revolutions per minute, we recommend adding enough ceramic media to bring it up to the 2/3 full level regardless of what type of rock is being tumbled.

Next add two level tablespoons of 110/220 grit or 150/220 medium grit silicon carbide for each pound of rock and filler media. Then add water until the water line is just below the top of the rocks. Again, run the tumbler for seven days.

After seven days, open the barrel and once again, thoroughly clean all of the grit from the rocks, barrel, and lid. Your rocks should now have a smooth frosted surface. Once again inspect your rocks, choosing the ones ready to move on, discarding the bad ones, and rerunning step 2 if necessary.

STEP 3 - Fine Grit Grind / Pre-polish

The next step is a fine grit such as 600 grit or 500 grit silicon carbide. By this stage your rocks should be mostly finished. This stage doesn't not really grind the stones, but rather removes any scratches and smooths out blemishes. Once again make sure everything is perfectly clean before proceeding. Place your rock and any pellet fillers you need into the barrel, and add two level tablespoons of fine grit per pound of material. Then add water until it fills the barrel up to just below the top of the rocks. Tumble this for about seven days. Once again inspect your rocks and discard any bad ones. At this point your rocks should be extremely smooth, and some of them might even look slightly polished.

STEP 4 - Polish

The most exciting step! Putting a high shine onto your tumbled rocks. It is very important in this step to start with extremely clean rocks and equipment. A tiny speck of grit can ruin your polishing step. You might even want to use a nail brush to make sure no grit is stuck in any crevices or holes. Or you can put the cleaned rocks and pellets back into the tumbler with just water and a mild dishwasher soap and run it for a few hours. Some people even use a separate barrel that they use only for the polishing step!

Your rocks should also be fully ground. This step does not grind them, so they must be ready for the finally polishing.

Place the rocks in the barrel and add two level tablespoons of rock polish (cerium oxide or tin oxide can be used, but are very expensive-TXP aluminum oxide powder is an excellent choice for tumbling) per pound of material in the barrel. Ground wallet nut shells are a good filler at this stage. Add water to just below the top of the rocks. Then, close the barrel, check for leaks and run for seven days.

When you are finished your rocks should be bright and shiny. If the stones have scratches on them, then you will need to go back to STEP 2 and repeat the medium grind, fine grind, and polishing steps.

If the rocks are extremely smooth but not shiny or appear hazy you may have small particles of polish in micro-size crevices. You can try cleaning them by tumbling for an hour or so in soapy water. This is referred to as "burnishing."

To burnish, place the rocks and plastic pellets in the cleaned barrel with the normal amount of water, and add about 1/2 tablespoon of dishwasher soap or grated oil free nonabrasive soap (such as Ivory bar soap) for each pound of rock.