How to Polish Chert

Clean the chert to remove any dirt. Chert is hard -- 7 out of 10 on the Mohs hardness scale -- so it can be scrubbed with a sponge and hot soapy water. You can also use diluted acid, such as oxalic and muriatic to clean chert.

Evaluate the chert samples you want to polish. Remove any samples that are irregular in any way. The samples should be small and semi-round in order to polish well. The rocks going into the tumbler at the same time should be similar in coarseness and size.

Place the rocks into the tumbler barrel, you must fill the barrel at least halfway, but do not fill the container more than three-quarters full. Weigh the barrel. Start with a coarse grit, using the chart included with your rock tumbler or from a chart found online to determine the correct amount.

Add water until it barely reaches the top of the chert samples in the barrel. Dry the edges of the barrel with a paper towel. Tightly seal the barrel and place it in the tumbler, then start the machine and leave for one week.

Stop the machine after one week and check your samples. The slurry that has accumulated must be removed. Place it in a disposable container and throw it away -- do not put down the drain. Rinse the rocks off with water until there is no grit left on the chert. If you like the shape the rocks have taken, move onto the next grit solution, but if not, repeat the coarse grit tumbling process. The rocks should still be in good condition, and if not, remove the irregular samples.

Place the rocks back into the barrel and add the correct amount of fine grit. Once again, fill the container with the appropriate amount of water. Once again, leave the machine running for one week, checking it each day to make sure it is still running.

Stop the machine and check your samples. The rocks should be shiny and smoother than before. Because chert is tough, you may need to repeat the process multiple times. Patience is key when rock tumbling.

Examine the rocks and ensure they are to the smoothness and shape you desire. Clean and dry the barrel completely before starting, along with the chert samples. Place the clean rocks into the barrel. Once again your rocks should fill the barrel between one-half and three-quarters high -- if not, add plastic pellets, which are found online or in craft stores, until it fills the container at least halfway. Add the appropriate amount of extra-fine grit and fill with the same amount of water. Set the machine to run for one week.

Stop the machine and remove the chert from the tumbler. Check to see if the chert is ready for polish by testing a spot on one of the samples with polishing powder on an old towel. If the sample shines, it is ready, but if it does not have a shine, it needs to repeat the extra-fine grit tumble.

Clean the barrel and chert samples and replace them in the barrel. Combine with plastic pellets if necessary. Add the correct amount of polishing powder and run the machine for 10 days.

Clean the polished rocks. You can do this by hand or with the rock tumbler. Use a gentle bar soap and water mixture and let soak or tumble for 24 to 48 hours. Rinse with water and dry.