"I tumble all my obsidian in the 60-90 (coarse) grit in a rotary tumbler until I have all the chips and pits out. I change grit each week and take out the stones that are without pits or chips. (remember course grit breaks down and needs to be freshen on a timely basis). I do not use any media in this stage. I put the stones back in fresh 60-90 grit that need more time. I go to 220 grit in the vibratory tumbler with ceramic media if I'm using my Diamond Pacific Mini-Sonic tumbler (MT-4). I tumble the batch for three days. I take any chipped or pitted stones out and progress to 600 grit for three days with ceramic shapes. I check for chipped or pitted stones before moving to the 1000 grit. I run the batch in 1000 grit for two days. I then put the batch in Rapid Polish for two days after I've removed any stones that have chips or pits. I run the load for two days in Rapid Polish. I then put the batch in tin oxide for a day or until they have a mirror finish. I slow down my tumbler just a little on the polishing stages. This is not so critical in the grit stages. Make sure to keep the rocks wet, but not too wet. If you are using the Mini-sonic four pound tumbler, the rocks hold moisture longer than in a bowl type machine. Follow the instructions in the user manual for the Diamond Pacific tumbler as far as grit and water are concerned. We put in enough ceramic shapes to fill the empty spaces as the rocks tumble. I would guess the load would have around three pounds of stones and one pound of ceramic shapes. Add water sparingly when the slurry starts to dry out. Do not let the stones get dry. This seems to be more critical in the polish stages with the Rapid Polish and tin oxide. We run the stones in shaved Ivory soap and water for a couple of hours to clean off the polish and add extra shine when tumbling in the tin oxide is finished. It only takes a little soap, just enough to make thick suds so it feels like lemon meringue. For a four-pound hopper, we add about two teaspoons of water and 1/2 teaspoon of grated soap to the polish slurry."
Gary, thank you very much for sharing this information. Your months of trial and error in learning to get a wonderful polish on obsidian is most appreciated.