Grit stages, explained
Every tumbling step uses a finer abrasive than the one before. Each grit removes the scratches left by the last, until nothing is left to remove but shine. Here is what each grit does and when to move on.
Smaller numbers, bigger grains
Numbers mean mesh
Grit is sized by sieve. A 60/90 grit passes a 60 mesh screen and is caught by a 90, so lower numbers mean coarser grains.
Silicon carbide cuts
Silicon carbide is harder than almost any stone you will tumble. Its grains break into fresh sharp edges as they wear, so they keep cutting.
Aluminum oxide polishes
The final step uses a micron-sized powder. It doesn’t cut so much as smooth the surface until it reflects light.
Where to start
Start with the coarsest grit your rock needs. Every step after that is the same for most stones.
36 extra coarse: very rough, angular rock or large pieces that need heavy shaping.
46/70 coarse: fast shaping for hard rough rock, with a slightly finer finish than 36.
60/90 coarse: the standard first step for agate, jasper and quartz.
Already smooth stones, like beach pebbles without pits, can often start at 120/220.
How long each stage runs
Typical times in a rotary tumbler, using about 2 tablespoons of grit per pound of rock. Vibratory steps run 1 to 3 days.
1 · Coarse
2 · Medium/fine
3 · Pre-polish
4 · Polish
When to move on
After coarse, every stone should be the shape you want, with no pits or rough spots. Most later problems start here.
After medium/fine, stones should be smooth and evenly matte when dry.
After pre-polish, they should have a soft satin sheen.
After polish, dry a stone and check it in good light. Hazy spots usually mean leftover coarse grit.
Don’t rush the coarse step
Finer grits can’t fix shape. If a stone still has rough spots, run coarse again with fresh grit.
Silicon carbide or aluminum oxide?
Silicon carbide
Sharp, very hard grains for the coarse, medium/fine and pre-polish steps. Cuts fast and keeps cutting as it breaks down.
Aluminum oxide
A fine white powder for the final step. Brings up a glassy shine on agate, jasper and quartz.
Not sure which grit you need?
Tell us what rock you have and what tumbler you run. We’ll tell you where to start and how much grit to use.