Guide
How I tumble rocks
This is the question I get more than any other in the comments, so here's the full answer in one place. Rock tumbling is simple in concept — run stones through progressively finer grit until they're smooth and shiny — but getting a good result depends on doing each stage properly and not rushing it.
The tumbler I reach for most: Thumler's Model B, a 15lb rotary tumbler. I've also reviewed budget and vibratory options — see the gear page.
The four stages
A rotary tumbler runs continuously — rocks and grit and water tumble together in a sealed barrel, and the grit does all the work. Each stage uses a finer grit than the last, and you have to fully clean the barrel and rocks between stages so coarser grit doesn't contaminate the next round (a few stray coarse grains will put scratches in an otherwise-polished batch).
- 1
Coarse Grind
Round off the rough shape and knock down any sharp edges, cracks, or pits.
Get this grit → - 2
Medium Grind
Remove the pits left by the coarse grit and smooth the surface further.
Get this grit → - 3
Fine Grind
Get the surface smooth enough that it starts to take a faint shine on its own.
Get this grit → - 4
Pre-Polish & Polish
This is where the shine actually shows up — micro-alumina is the final polish stage.
Get this grit →
Timing is a starting point, not a rule — rock hardness, tumbler load, and how coarse your rough material is all shift these. Open the barrel partway through a stage and check progress; if it's not there yet, keep running it.
Things I've learned the hard way
Sort by hardness
Don't mix very soft and very hard rocks in the same batch — the soft ones can get ground away before the hard ones are even shaped. Run separate batches when you can.
Cracks and fractures aren't disqualifying
A rock with holes, fractures, or other imperfections can still tumble into something worth keeping — it just takes longer, and won't always come out flawless. Worth trying before you toss a rock that looks rough.
Watch: can you tumble rocks with holes and fractures →Maintain the tumbler
The lid gasket is the part that gets people — let it wear too thin and you'll find grit slurry everywhere. Replace it before it fails, not after (see the gear page for the part).
Clean thoroughly between stages
Wash the rocks, the barrel, and yes, your hands, between every stage. Leftover coarse grit is the single most common reason a "polished" batch comes out scratched.