Once you have enough rocks for the extra fine grind/prepolish step, place them in a barrel that has been thoroughly cleaned. (If you do not clean the barrel and stone thoroughly a few pieces of grit from the previous step will contaminate your pre-polish - that will produce scratches on your finished gemstones!)
At this step some people use plastic tumbling pellets to cushion the rocks. After your barrel is about 2/3 to 3/4 full of rocks and pellets add water to slightly below the level of the rocks. Then add the extra-fine grit (sometimes called pre-polish) as shown in the chart below, seal the barrel and you are ready to tumble.
Run the prepolish step 24 hours a day for 7 days. When you open the barrel, the rocks, slurry and pellets will look like a funny tapioca (see image below).
Here are our recipes for different size tumbler barrels. If your barrel size is not listed here you can estimate the amount needed or consult the instructions that came with the tumbler.
Now, gently pour the contents of the barrel into a collander. Rinse the rocks over a bucket or disposable container. (Remember: donít pour the mud or used polish down a drain!)
Safety glasses are highly recommended when working with grit, polish and tumbler slurry or opening tumbler barrels.
To easily separate the rocks from the pellets, you can put them in a small container of clean water. The pellets will float to the top and can be skimmed off. The rocks should have a slight luster when dry.
To make sure that the rocks are ready for polishing, perform this simple test: take an old towel or piece of felt and barely dampen it. Sprinkle a small amount of polishing powder on it 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, or if the rocks donít show luster when dry, you should repeat the prepolish step, checking every couple of days, until the desired results are achieved.
Step 4: Polish
Clean the barrel, lid, and stones as before. Charge the barrel with stones, the appropriate amount of rock polish as indicated in the table above, and water. Add plastic pellets for cushioning until the barrelís volume is 2/3 to 3/4. 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, you may want to try burnishing them to see if they will gloss up a bit.
Step 5: Burnishing
Just as before, clean the barrel, lid, and stones. You should be an expert at clean tumbling by now. Then, gently place the stones back in the barrel, and add soap according to the table above. Classic Ivory Soap is recommended - do not use a soap that contains abrasive particles. 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. Add clean plastic pellets and water. Allow this mixture to run for 1-2 days.
When you open the barrel at the end of the burnishing step you will find a thick froth of suds. Under the suds are your sparkling gems. Rise them with clean water, then dry them off.
This video shows you how to determine if your rocks can benefit from "burnishing" and demonstrates how it is done. See more videos and rock tumbling instructions at RockTumbler.com, a partner site of Geology.com.