The four-step rotary tumbling guide
This is the schedule we use for most agate, jasper and quartz in a rotary tumbler. Plan on about four weeks from rough rock to finished polish.
Load the barrel
Fill the barrel two-thirds to three-quarters full. Too empty and stones crash into each other. Too full and nothing moves.
Mix sizes. Small stones fill the gaps between larger ones and help the grit reach every surface.
Only tumble stones of similar hardness together. Soft stones will wear away next to agate.
Short on rock? Top up with ceramic spacers instead of running a half-empty barrel.
Add water until it sits just below the top layer of stones.
Steps 1 to 4
Coarse grind with SiC 60/90
Shapes the stones and removes pits and sharp edges. Open the barrel after a week. If stones still have rough spots, rinse and run another week with fresh grit.
Medium/fine with SiC 120/220
Erases the scratches from step one. Stones should come out smooth and evenly matte when dry.
Pre-polish with 500
Leaves a soft satin surface. Plastic pellets cushion the load so stones don't chip as they get smoother.
Final polish with aluminum oxide
Brings up the shine. Use a separate barrel for polish if you have one. A single grain of coarse grit can haze a whole batch.
Between steps
Clean everything before the next grit goes in. Rinse the stones in a colander, then scrub the barrel and lid with an old toothbrush. Check stones for cracks and pull any that are broken, since they'll scratch the others.
Never pour slurry down the drain
Grit slurry settles in pipes and sets like concrete. Pour it into a bucket, let it dry, and throw it out with household trash.
Steel shot for jewelry and metal
Stainless steel shot burnishes metal instead of grinding it. Load about 1 lb of shot for every 1 lb of jewelry, cover with water, and add a few drops of burnishing compound or plain dish soap. Run 1 to 4 hours, then rinse and dry. Store the shot dry or in clean water so it stays bright.
Common problems
Stones came out hazy after polish
Usually leftover coarse grit. Clean the barrel well, then rerun step 3 and step 4.
The barrel leaks
Wipe the rim and lid dry before closing. Grit on the seal stops it from sealing.
Gas buildup or a swollen lid
Some stones release gas as they grind. Open the barrel every day or two during step one to let it vent.
Stones are chipped or bruised
The barrel is too empty, or the stones are too soft for the load. Add spacers or plastic pellets.