Rock Tumbling Basics

Rock tumbling involves taking rough rock pieces and tumbling them with a series of grits in a container (barrel) so that they emerge as polished, smooth, shiny stones. It is a fun and fascinating process, especially for children, and a great way to introduce them to rocks and lapidary in general. The instructions here are for rotary type tumblers. Be aware the entire process will take 4 to 6 weeks to complete.

PREPARATION | STEP 1 | STEP 2| STEP 3| STEP 4| HELPFUL TIPS | COMMON QUESTIONS

Supplies Needed

• Rotary Rock Tumbling Machine: This is a machine that turns barrels as opposed to a vibratory type machine which uses either ultrasound or spin around a vertical axis. Rock tumblers are are designed to turn a specific barrel weight and some machines are designed to turn more than one barrel at a time.  Properly maintained a rock tumbler can last for years. Look for a machine with a good reputation, sound construction, and a warranty. Avoid the toy store plastic rock tumbler or polishers.
• Tumbler Barrel: These hold the rock as it turns on the machine. Most barrels today are a thick rubber barrel or rubber lined barrel with a softer, removable lid that cuts down on noise, acts as a cushion, and holds up to the grinding action. They are designated by the weight they will carry, commonly found sizes include: 1.5, 3, 4.5, 6, and 12 lb, the most common being 3 and 6 pound. To avoid contamination you may consider using one barrel per step (4 required) or one for coarse grit steps, one for polishing steps (2 - 3 needed).
• Grits: Silicon carbide is most commonly used. Grit is gauged by the size of the particles and different size particles are used in the various steps to perform specific tasks in the tumbling process. You will need a minimum of three grits: 60/90 (coarse), 120/220 or 240/400 (medium), and then a 400 to 600 (fine/pre-polish). Most silicon carbide grits weigh about one ounce per slightly-rounded tablespoon.
• Polish: This is used in the final step to create that "wet" look. Cerium oxide polish is commonly used for polishing quartz, glass, obsidian, apache tears, and agate. Other choices including aluminum oxide (suitable for quartz, agate, and harder stones) and tin oxide.
• Tumbling Media / Filler: This is non-rock material placed into the barrel of your tumbler to cushion the rocks from impact and/or to serve as a filler to keep the volume in the barrel at optimal level. They are also used to balance particle size in a tumbling load and to improve the tumbling action. Common filler materials include plastic pellets and ceramic media. Ceramic is primarily used as filler to bring the barrel to proper operating capacity as it stands up to the grinding process. Plastic pellets are most often used in pre polish and polish steps for cushioning as well as a filler. Another option is to use pre tumbled material at the same stage as fill when needed.
• Rocks Rough: Start with good rough! Look for rocks with interesting colors and patterns. See the table below for suggestions. Many lapidary suppliers sell bags of rough in single variety or a suitable mix of stones already broken into sizes for tumbling. Pieces should be no larger than ˝ of the diameter of the barrel you are using and a mix of sizes is best. Pieces any larger than this will not fall correctly and will not polish to the desired high gloss.

Preparation

1. Your rough must be clean and free of grit, debris, dirt, oils, etc. You can clean them by hand using a scrub brush and soapy water or use your tumbler. To tumble them clean, fill the barrel 2/3 -3/4 full with rough, cover with water until just below the top layer of rock and add a dishwashing detergent like Dawn (1 teaspoon for barrels under 3#,  1 tablespoon for larger 3-6#). Put the lid on the barrel and tumble for at least 3 hours but no longer than overnight. Whichever method you choose make sure to always do any cleaning or rinsing so that you collect  all waste into a separate container. A 5 gallon bucket with a plastic sieve or strainer that fits over your bucket works well. NEVER send any slurry, grit, waste water, or anything from cleaning or tumbling down your household drains!

2. Remove any rocks that have obvious flaws such as pitting or weak fractures. You can break rocks with fractures on the fracture planes (safety first!) and then use them. 

3. Next sort your stones by hardness. You will get the best results when you tumble stones of similar hardness. Mixing rocks of different hardness results in softer stones being ready for the next step sooner than harder ones and incomplete polishing. Use a Rock Guide and Moh's Scale to identify and sort your rough into batches of similar hardness.

4. Then sort stones of similar hardness by size so that you can achieve a balanced load. A balanced load has approximately equal numbers of large, mid, and smaller size stones. For example: 33% small rocks (1/4 to 1/2 inch), 33% medium rocks (1/2 to 1 inch) and about 33% large rocks (1 to 1 1/2 inches). Stones of mixed sizes will rub against each other at more points and grind faster and more effectively; a batch of rocks all about the same size will likely not tumble properly or grind very slowly.

5. If you want to end up with certain shapes or sizes you can preform your rough using a rock hammer (safety first!) or by shaping them using a lapidary saw or a diamond router like the SwapTop Grinder / Shaper.

6. The best tumbling action happens when the barrel is filled 2/3 to 3/4 full (~70%). If you have too much in the barrel, greater than 75%, you will actually “lock” the rocks in place and they wont tumble. Too little in the barrel, less than 60%, and you will likely end up with a sub-standard polish after going through the entire process.

7. The amount of grit you will need depends on the size of the barrel, the type, and the hardness of the stones you are tumbling. The table below suggests a range of amounts to get you started. Experience and knowing your material will allow you to zero in on the right amount for the results you want.

STEP 1: Coarse Grind

1. Fill yourbarrel with rough to 70% full and add the grit. If your rocks are chips or have rough and sharp surfaces you will start with a course grit 60/90. If your rocks are already worn and kind of rounded, say from being in a stream or ocean, skip to STEP 2: Medium Grind.

2. Add hot water to the barrel until it is just below the top layer of rocks. You should see water between the rocks, but not covering them (about 90% of the stone is completely submerged). Using hot water will help to seal the barrel top. Add ˝ teaspoon of liquid dishwashing detergent (like Dawn), secure the lid, and place the barrel on the tumbler.

3. Put the tumbler into operation according to your model's instructions, checking to make sure all belts and pulleys are operating properly. Listen to the sound coming from inside the barrel. You should hear a slushy, tumbling sound as the barrel turns, moving the grit and rocks around in the barrel. If you don't, stop the machine, open the barrel and inspect the consistency of the mixture and if too dry, add a bit of water. Secure the lid and replace the barrel on the machine. When all seems to be operating smoothly, leave the machine to run checking back periodically to make sure it is operating smoothly.

4. It will an average 5 to 7 days (120 to 168 hours) of running with the coarse grit to remove the sharp and broken edges on your stones. Agates can require between 9 to 10 days (or more) of coarse grinding. Experience and keeping a record of your batches will help you make this decision. If your stones are still not rounded to your satisfaction it is may be the grit has broken down and is no longer working effectively. Dump the slurry off the rocks (into the bucket, not down the drain!), add more water and grit as you did above, and continue tumbling. You may find that some agates and jaspers need to go through the coarse grind step 2 or more times depending on their hardness and the amount of roundness you desire.

5. When you are satisfied with the results of your coarse grind, dump the entire contents from the barrel into the sieve over your bucket.

6. Rinse the stones, barrel, lid, and your hands, over the bucket to remove all traces of grit. DO NOT wash where the waste will go down your household drains; it can clog and harden in traps or pipes! Put the rocks back into the barrel and fill it with hot water until the stones are covered. The volume of the stones will have dropped but should still be over 60% full. Add one tablespoon of liquid detergent (Dawn). Close up the barrel and place it back on the tumbler for at least 3 hours but not over 24 hours. Open the barrel and again empty the entire contents into the sieve over your bucket. Carefully wash the entire barrel, lid, and carefully rinse off the rocks to remove all traces of the grit and residue before moving on. Failure to do so can produce scratches in the next step that won't be removed or will be polished into the stone. (see Helpful Tips #2)

STEP 2: Medium Grind

1. Place your clean stones back into the clean barrel. As your stones tumble, they will lose approximately 30% of their size and at some point they will drop below the optimal 70% full point in the barrel. At this point you need to add a filler material to make up the volume. You can use ceramic beads, plastic pellets, or other rocks from a previous tumbling that are at step 2.
2. Add the 220/240 or 240/400 finer grit to the barrel. Add hot water to the barrel until it is just below the top layer of rocks. You should see water between the rocks, but not covering them (about 90% of the stone is completely submerged). Add ˝ teaspoon of liquid dishwashing detergent (like Dawn), secure the lid, and place the barrel on the tumbler.
3. Put the tumbler into operation according to your model's instructions, checking to make sure all belts and pulleys are operating properly. You should hear a slushy, tumbling sound as the barrel turns, moving the grit and rocks around in the barrel. If you don't, stop the machine, open the barrel and inspect the consistency of the mixture and if too dry, add a bit of water. Secure the lid and place the barrel on the machine. When all seems to be operating smoothly, leave the machine to run checking back periodically to make sure it is operating smoothly.
4. It will take about 4 to 7 days (96 to 168 hours) of running with medium grit to further round and smooth the stones. Remember softer stones = shorter time, harder stones = longer time.
5. When you are satisfied with the degree of your medium grind, dump the entire contents from the barrel into the cleaned sieve over your bucket. Rinse the stones, barrel, lid, and your hands, over the bucket to remove all traces of grit. DO NOT wash where the waste will go down your household drains; it can clog and harden in traps or pipes!
6. The volume of the stones will have dropped but should still be over 60% full. Add one tablespoon of liquid detergent (Dawn). Close up the barrel and place it back on the tumbler for at least 3 hours but not over 24 hours. Open the barrel and again empty the entire contents into the sieve over your bucket. Carefully wash the entire barrel, lid, and carefully rinse off the rocks to remove all traces of the grit and residue before moving on. Failure to do so can produce scratches in the next step that won't be removed or will be polished into the stone. (see Helpful Tips #2)

STEP 3: Fine Grind / Pre-Polish

1. Extreme care must be taken from this point forward to prevent your nearly finished stones from chipping or scratching. At this point they should be smooth and rounded but not shiny.
2. Put an inch or so of water into the barrel first and then gently place the stones being careful to avoid chipping and scratching them. If they do not fill the barrel to the proper level you will need to add a filler until you reach your 70% fill level. DO NOT use any filler from any of the previous steps as it is contaminated with that grit!
3. Add the desired pre-polish grit size to the barrel. Add hot water to the barrel until it is just below the top layer of rocks. You should see water between the rocks, but not covering them (about 90% of the stone is completely submerged). Add ˝ teaspoon of liquid dishwashing detergent (like Dawn), secure the lid, and place the barrel on the tumbler.
4. Put the tumbler into operation according to your model's instructions, checking to make sure all belts and pulleys are operating properly. Listen to the sound coming from inside the barrel. If all seems to be operating smoothly, leave the machine to run checking back periodically to make sure it is operating smoothly.
5. It will take about 5 to 7 days (114 to 168 hours) of running with the pre-polish grit to get them ready for polishing. When finished, carefully pour the entire contents from the barrel into the cleaned sieve over your bucket. Don't just dump them out as you can chip or scratch your stones, negating all the work you've done this far! Carefully rinse the stones, barrel, lid, and your hands, over the bucket to remove all traces of grit. DO NOT wash where the waste will go down your household drains; it can clog and harden in traps or pipes!
6. Put the rocks back into the barrel and fill it with hot water until the stones are covered. Add one tablespoon of liquid detergent (Dawn) and close up the barrel. Place it back on the tumbler for at least 3 hours but not over 24 hours. Open the barrel and again empty into the (cleaned) plastic strainer over your bucket. Carefully wash the entire barrel and carefully rinse off the rocks to remove all traces of the grit and residue before moving on. Failure to do so can produce scratches that will be polished into the stone! (see Helpful Tips #2)

STEP 4: Final Polish

1. Allow your stones to dry and then examine them carefully. Remove any stone that is chipped, broken, has sharp edges or does not appear to have the same degree of finishing. Leaving these in will damage the rest in the batch; save them to use as filler in a future tumble.
2. Put an inch or so of water into the barrel first and then gently place the stones in being careful to avoid chipping and scratching them. If they do not fill the barrel to the proper 70% full level you will need to add filler material. DO NOT use any filler from any of the previous steps as it is contaminated with that grit, it must be new!
3. Add the cerium oxide, tin oxide, or aluminum oxide. Add hot water to the barrel until it is just below the top layer of rocks. You should see water between the rocks, but not covering them (about 90% of the stone is completely submerged). Add ˝ teaspoon of liquid dishwashing detergent (like Dawn), secure the lid, and place the barrel on the tumbler. Put the tumbler into operation according to your model's instructions, checking to make sure all belts and pulleys are operating properly. If all seems to be operating smoothly, leave the machine to run checking back periodically to make sure it is operating smoothly.
4. It will take between 10 to 15 days (240 to 360 hours) of running to bring your stones to nice shiny polish. Again, less time for softer rocks, longer for harder ones. When you are satisfied with the degree of polish, very carefully remove the entire contents from the barrel into the cleaned sieve over your bucket. Don't just dump them out! Carefully hand wash the stones along with the barrel, lid, etc. over the bucket to remove all traces polish. DO NOT wash where the waste will go down your household drains; it an clog and harden in traps or pipes!
5. Spread your washed stones out on a cloth or towel to dry. Stones that are properly polished will have the same look dry as they did wet. Any stones not polished to your satisfaction can be run again with another batch as filler. Congratulations, you're done!

Helpful Tips

1. Tumblers are rated by weight capacity. That means that a 3 pound tumbler will rotate 3 pounds of total weight indefinitely. That weight rating is the total of the barrel and rock combined.
2. You can reduce the amount of cleaning time and effort by purchasing a barrel for each grit step. Using separate spoons, sieves, and barrels for each type step can also help avoid cross contamination of grits. Modern barrels do clean out very easy so it is not a problem to clean between steps, just be meticulous.
3. A 5 gal bucket (or two) works well for collecting the waste. When one gets somewhat full let it sit and settle for 4 to 6 days. At that point you can pour off the supernatant (liquid portion). Let the remaining residue dry out and then empty it into the trash. NEVER pour the grit, slurry, or unsettled liquids down your drains!
4. Using a liquid dishwashing detergent such as Dawn does produce a sudsing foam which acts as a cushion for your rocks. It also provides a surfactant which makes your water “wetter” and assists the grinding action of your grits.
5. Do not reuse the grit. Grit breaks down and is useless after the prescribed grinding time.
6. Make notes of each tumble you do - rock type, how long each step took, grits, and amount used. Experiment with the amount of grit you use at each step and track the results. This can save a lot of time and money in the future.
7. When tumbling more than one barrel at the same time note the start date and step of each so you how much time has passed at that particular step for each barrel.
8. A syringe is useful when oiling your tumbler to avoid applying too much and having oil splash all over when you restart.
9. Take care of your machine and it will take care of you! You will need to oil the motor and bearings according to your machines manufacturer's recommendation. Keep a log of service steps you perform.
10. Safety first! When breaking up stones always wear protective goggles! Wrapping pieces up in leather or towels before striking them with a hammer will protect you from flying shards (many semi-precious gemstones virtually explode when hit hard enough) and catch all those tiny shards.

Common Rock Tumbling Questions

What are the best types of rock to polish?

Any rock with a Moh's hardness of 5 to 7 will generally polish well in a tumbler; Agates and Jaspers are very suitable and come in many colors and patterns. Generally, if the rock rough has a glassy luster will tumble to a shine well; if it has an earthy luster it will likely have an earthy luster after tumbling. Here are some commonly tumbled stones:

Quartz Varieties for Tumbling				Other Commonly Tumbled Stones
Stone	Moh's #	Stone	Moh's #	Stone	Moh's #	Stone	Moh's #
Agate	6.5 - 7	Jasper	6.5 - 7	Amazonite	6 - 6.5	Moonstone	6 - 6.5
Amethyst	7	Petrified Wood	7	Hematite	5.5 - 6.5	Obsidian	5 - 5.5
Aventurine	6 - 6.5	Rose Quartz	7	Lapis Lazuli	5 - 6	Opal	5.5 - 6
Carnelian	6.5 - 7	Tiger's Eye	6.5 - 7	Malachite	3.5 - 4	Turquoise	5 - 6

Compatible Tumbling Materials*
Your best results will happen if you tumble a single rock type in a batch, that is all amethyst or all agates but if you want to mix up your load, below are groups of stones that generally tumble similarly and with good results. There are stones that must be tumbled alone - those listed in best tumbled alone. These should never be run in a mixed batch, even with those listed in the group.
Soft:	Amber, Chrysocolla, Malachite, Malachite/Azurite, Serpentine, Rhodochrosite, Varisicitle, Smithsonite
Soft + Brittle:	Amazonite, Apatite, Azurite, Common Opal, Labradorite, Lazulite, Lazurite, Moonstone, Peristorite, Rhyollite
Medium Hard:	Diopside, Howlite, Jadeite, Olivine, Prehnite, Pyrite, Pyrolucite, Rhodonite, Sodalite, Unakite,
Hard + Crystalline:	Amethyst, Aquamarine, Citrine, Rose Quartz, Rutilated Quartz, Smokey Quartz, Quartz (Rock Crystal), Tourmalated Quartz,  Tourmaline
Hard:	Adventurine, Brazilian Agate, Agatized Coral, Carnelian, Chalcedony, Dendritic Agate, Epidote, Fire Agate, Jasper, Lace Agate, Moss Agate, Petrified Bone, Petrified Wood, Plume Agate, Sagenite Agate, Tiger Eye
Very Hard:	Ruby, Sapphire
Best Tumbled Alone:	Apache Tears, Beryl, Cullet (glass), Garnet, Goldstone, Obsidians, Spinel, Topaz

What types of rock can't be tumbled?

• Sedimentary rocks like sandstone, coal, limestone and shale are too soft or poorly cemented to polish into shiny gems.
• Metamorphic rocks that contain micas or have a "grainy" texture (slate, schist, gneiss) will break up instead of becoming smooth.
• Most Igneous rocks contain several different minerals that wear down at very different rates and wont tumble uniformly.
• Rocks with protrusions, voids or fractures don't tumble well as they tend to break during the finer grit and polishing steps and end up scratching the others in the batch. Try breaking them into smaller pieces with a rock hammer (safety first!) or just don't use.

My stones have a more matted, not glossy and shiny. Why?

• Impatience. If you rush the coarse and medium grind steps you can be leaving scratches in the rocks that can't be removed in the later polishing steps.

• You may have introduced contaminants between steps. Make sure that you clean rocks, barrel, and seals extremely well between each step. Never use filler from one grit step in the next.

• Another possibility is that you let the rock level in the barrel fall below 1/2 by not using enough filler material to provide cushioning.

• Some rocks, such as sandstone, calcite, onyx, and limestone, just won't take a high polish in a tumbler and if your rocks are made from minerals of different harness, only the hardest minerals in the stone will polish.

Can I reuse the plastic pellet filler?

Yes but you can only reuse for the step they were originally used with. The grit gets imbedded in them and if you use them in a different step and / or with a different grit you will contaminate the load. They can be used about 10 to 12 times before they are no longer effective.

Why are there white lines in my polished stones?

These lines are from polish getting into small cracks in the rocks. An Ultrasonic cleaner will generally remove this. You can also try running your rocks again through an extra cleaning tumble with the addition of plastic pellets to the problem rocks and soapy water to resolve this problem. With everything clean, carefully return the stones to the clean barrel along with new filler. Add one tablespoon of liquid detergent (Dawn) and water to make a thick, soapy solution. Start by running the barrel for a couple of hours and check. If the lines are gone you are done. If not, continue on for a few more hours and check. If after 24 hours of running you still have lines they won't be removed by this method.

Why did only the edges get polished?

Your load was likely far below the 50% minimum.

How big a rock can I polish in my barrel?

In theory, as large a rock as will fit in the barrel with some wiggle room (remember, it needs to "tumble") and that doesn't put it over the weight rating of your barrel. But the 50% rule, that is the rock diameter is no more than 50% of the barrel diameter, is the best insurance to secure a good polish. If you want to do one large rock, put it in the barrel and fill will gravel or small pebbles to 70% full. Note that you might need to repeat the coarse grind step multiple times to get the roundness on the big stone you want.

Why did only the centers get polished?

Either your slurry was too thin or you didn't use enough filler to carry the abrasive.

How much rough should I buy?

On average 30% of the rock or more will be ground away in the coarse grind step. If you process two barrels of rough through the coarse grind you will get about the right amount of rock to finish through the process with a single full load.

Can I use other types of soaps in the cleaning?

There is no true laundry soap any more. Ivory Snow flakes became a detergent in the late 1990's. All of the laundry detergents including Ivory Snow flakes contain “brightners” which will dull your rocks. They actually react with some of the rocks and can cause the barrels to leak. The only true soap left on the market is bars of Fels-Naptha soap and difficult to find. If some rocks end up not polished to your satisfaction, use them as fillers in future runs.

What alternatives are there to purchased filler?

Inexpensive marbles, plastic pellets, or crushed walnut shells can be used as filler in grinding steps. Sawdust, wood shavings, cornmeal, walnut shells, or rubber strips (such as cut up rubber bands) are alternatives used by some in the pre-polish and polish steps.

How can I get specific shapes?

It is possible to preform your rough and then run it through the tumbling process to produce stones with a specific shape in the end. The desired final shape is cut out first using a diamond saw or on a grinder / shaper. You will need to grind smooth and remove saw marks, heels, nicks, and grooves before tumbling. They should be tumbled with other stones of exactly the same type. Also realize that the final shape will be a "rounded off" version of the original.

Can you tumble polish slabs?

Slabs can be tumbled in similar materials of equal hardness. Larger diameter slabs will be more difficult to polish than smaller because the larger the flat surface, the less contact points. Keep the percentage of slabs to under 20% of the total batch volume. You will also have to make sure there are no broken stones anywhere in the batch as any broken piece will always end up scratching the flat surfaces.

A tumbler runs for such a long time, how much electricity does it use?

A 3# rock tumbler uses about 10-20 watts of electricity. If you pay $0.20 per KWH it would cost you about $2.88 worth of electricity per month.

Wattage x 1 (hour) = A, the kWh of electricity used per hour by appliance
  1000

then take A  x your cost per KWH x 24 hours in a day x number of days you run the machine.

Here: [20 watts/1000 x 1]  x [$ .20 (cost)] x 24 x 30 days = $2.88

Compiled with assistance from Ron Rolfe from the Mt. Clemens Gem & Lapidary Society
* The Complete Book of Rock Tumbling, Christopher S, Hyde and Richard A Matthews