Steps for Rock Polishing and Rock Tumbling Grit

Over the years, as stones in the river or ocean get tossed around by the waves and rolled against each other and against the sand, they lose their rough edges and become smooth. A rock tumbler recreates this natural process and transforms rough stones into beautiful, semiprecious gems much faster than Mother Nature ever could. A rock tumbler can even add a high gloss if the last step in your traditional tumbling process uses titanium dioxide- the same material used in making toothpaste. There is a wide selection of polishes and burnishing agents available today, and each one is ideal for use with a certain type of rock. For best results in either a rotary or vibrating rock tumbler, you'll need a coarse grit as well as two or three finer grits. Each level of grit will need its own stage in the tumbler. It's generally more cost effective to buy tumbling grit in a kit. Lortone rock tumbling grit is very popular, but other rock tumbler grit kit brands will be less expensive. 

Steps for Rock Polishing. Be sure to following the instructions included with the specific rock tumbler you purchased when buying rock tumbler supplies. In general, you start with "ugly" rocks known as tumbling rough. Most lapidaries first run a cycle with the coarsest grit, and work down to the finest grits, and then use a polish for the final stage. Be sure your rough is all about the same size and hardness. Don't reuse grit because it breaks down and becomes useless after about a week. But you can reuse the plastic pellets that are added during the pre-polish and polish stages. This filler, which you can reuse 10 or 12 times, helps cushion the stones and reduces chipping. Remember to use the pellets with only one grit because the abrasive will become embedded in the pellets and if you reuse them in a different grit, you'll contaminate that load. Preventing contaminants from entering the process will help ensure a good polish.

Step 1 - Coarse Grind. Place your rough rocks into the barrel with coarse silicon carbide abrasive (60/90 grit) and water.  The barrel must always be at least half full for the tumbler to operate properly, but take care not to overload. Barrels more than 3/4 full (including grit and water) may be too heavy and burn out the motor, and might not allow enough space inside for the rocks to tumble.

Add the recommended amount of coarse grit and then add enough water to reach the bottom of the top layer of stones, but not so much that they are covered. You want to see some of the rocks above the water. Clean the inside and outside rims of the barrel and the edges of the lid. Make sure the surfaces that will form the seal are clean and dry. Place the lid on the barrel and seal the barrel. Put the barrel on the tumbler and start the motor. Wait a few minutes to make sure the tumbler is running smoothly and the barrel isn’t leaking. If the barrel is leaking, stop the machine, remove the lid and re-clean the surfaces of the lid and barrel that form the seal. Double-check that they are clean and dry. Replace the lid securely and proceed. Also, check the tumbler in a few hours again to make sure it’s still running smoothly. After this, it can probably be left unattended, but it’s not a bad idea to check up on it every day or so.

Let rotary rock tumblers run for 7 days, 24 hours a day. You can open the barrel occasionally to check the slurry (the muddy liquid inside), but if you do, be sure to clean and dry the seal areas before replacing the lid! After about a week, stop the tumbler and open the barrel. It will look like a barrel of mud! Rinse the rocks and inspect them. Just about all of the shaping is now done. The remaining steps only smooth the rock, not shape it, so the shape you see now is pretty much what you will end up with if you continue to the fine grind. If you would prefer your rocks to be more rounded than what you see now, repeat this step with fresh coarse grit until they are shaped to your liking.

If you are proceeding to the fine grind, first clean the rocks, barrel, and lid VERY thoroughly. Pay particular attention to the seal area and the bottom edges of the barrel . This cannot be stressed enough. If you have grit or slurry left in the barrel or on the rocks when you proceed to the next step, you will contaminate the load and might cause the barrel to leak.

Step 2 - Fine Grind. In this second step the rounded rocks are placed in the barrel with fine silicon carbide abrasive (120/220 grit) and water and tumbled for several days. The fine grit smooths the surface of the rocks and remove scratches left by the 60/90 grit. Again, add enough water to reach the bottom of the top layer of stones, but not so much that they are covered. If necessary, add plastic pellets to bring the volume of the barrel to at least half full. Check the seal area to be sure that it is clean and dry. Place the lid on the barrel and seal the barrel. Place the barrel on the tumbler and start the motor. Be sure the tumbler is functioning properly and the barrels aren’t leaking. If OK after a few minutes, check back again in an hour, and periodically after that. Let the fine grind run 24 hours a day for 7 days, then check the rocks. They should appear shiny when wet.

Step 3 - Pre Polish. In the pre-polish step, the smooth and rounded rocks are placed in the barrel with 500 grit aluminum oxide or 500 grit silicon carbide, plastic tumbling pellets, and water. Be sure to thoroughly clean the barrel and stones first. If you do not, a few pieces of grit from the previous step will contaminate your pre-polish and scratch your finished gemstones. Run the prepolish step 24 hours a day for 7 days. This step removes scratches left by the 120/220 grit abrasive and prepares the surface of the rocks for the final polish. Almost no shaping takes place during this step. The purpose of the plastic pellets is to cushion the rocks from impacting each other, and to fill the barrel if you are running a small load of tumbling rocks. Rinse the rocks after this step and inspect to make sure they are ready for polishing. To know for sure, sprinkle a small amount of polishing powder on a piece of old towel and rub one of the rocks vigorously over the powder. If you see a definite shine, the rocks are ready to proceed to the polishing step. If there is no shine, repeat the pre-polish step, checking the progress every couple of days.

Step 4 - Polish. In this step the clean rocks are placed in a clean barrel with a rock tumbler polishing powder such as aluminum oxide, tin oxide, or cerium oxide. Again, plastic pellets can be added during this step to cushion the rocks and to fill the barrel for small loads. Let the polish step run 24 hours a day for 10 days. The rocks should look shiny when dry. If the rocks have not polished to a high gloss or appear to have a film on them, try burnishing them to achieve a higher luster.

Step 5 - Burnishing (optional). If the finished rocks have the luster and shine you are looking for, then skip Step 5. If not, just as before, clean the barrel, lid, and stones. Place the rocks back in the barrel with clean plastic pellets, water, and a non-abrasive mild bar soap, such as Ivory. Grate or thinly slice the bar until you have the appropriate amount. Do not use liquid soap, as some of them contain oils that can harm the rubber barrel, break down the polish, and effectively un-polish your gems. Allow this mixture to run for 1-2 days. Burnishing usually removes any polishing film that was left behind, and yields brilliantly polished rocks.

You're done! Congratulations and enjoy your sparkling gems!