Four weeks is a long time to wait to find out you loaded the wrong rocks.
That's the real cost of a bad tumbling batch. Not the grit, not the electricity, not even the rocks — it's a month of running a machine every day and opening the barrel at the end to find pitted, cracked, dull stones that never had a chance. Almost always, the problem was decided in the first ten minutes, when the rocks went in.
So here's the stone-by-stone version. What works, what doesn't, and why.
The One Rule Everything Else Follows
Rocks in a tumbler don't just get polished by grit. They grind against each other, constantly, for weeks. That means the hardest rock in your barrel sets the terms for everything else in there.
Hardness is measured on the Mohs scale, 1 to 10. The sweet spot for tumbling is 6 to 7. That range is hard enough to take a real polish and survive the coarse stage, soft enough that ordinary silicon carbide grit can actually cut it.
Mix a Mohs 7 quartz with a Mohs 3 calcite and you have not made a varied batch. You have made a rock tumbler that spends four weeks turning your calcite into mud.
The practical test: everything in the barrel should be within about one point of hardness. Two points is pushing it. Four points is a guaranteed loss.
Tier 1: The Stones That Almost Can't Fail
If you're on your first few batches, use these and nothing else. They're forgiving, they're hard enough to shrug off mistakes, and they come out of the polish stage looking like glass.
Agate — the gold standard
Mohs 7, microcrystalline, no cleavage planes, essentially no porosity. Agate is what tumblers use to calibrate whether their process is working, because if agate doesn't come out shiny, the problem is you and not the rock.
The banding is the bonus. Those layers are a record of silica-rich fluid seeping into a volcanic gas cavity and depositing over long stretches of time — the chemistry shifted, the color shifted, and you get stripes. Polish reveals depth in those bands that you simply cannot see in the rough.
Jasper — agate's opaque cousin
Also Mohs 7, also chalcedony, but loaded with up to 20% other minerals — iron oxides, clay, organic material. Those impurities are what give jasper its color and pattern instead of translucent banding.
Jasper is arguably more forgiving than agate because it's opaque, so small internal flaws don't show. Our Mookaite Jasper from Western Australia is a good example of what finished jasper should look like — mustard, burgundy and cream, all in one handful.
The quartz family
Mohs 7 across the board: clear quartz, rose quartz, smoky quartz, amethyst, citrine. All the same mineral, different trace impurities.
One caveat worth knowing: quartz that formed as visible crystal points often has internal fractures where the points meet. Those open up on the coarse stage. Massive quartz — the cloudy, chunky kind — tumbles more reliably than pretty terminated crystals do.
Our Large Purple Amethyst Tumbled Stones show what well-run amethyst looks like at the end of a four-stage process.
Aventurine
Quartz with fuchsite mica inclusions that give it a soft internal glitter. Mohs 7, tumbles like any other quartz, and the shimmer gets noticeably stronger with polish. See the Large Green Aventurine Tumbled Stones.
Tier 2: Great Results, But Pay Attention
Petrified wood
Silica replaced the original wood cell by cell, so it tumbles as quartz — Mohs 7 — while keeping visible grain. The catch is that some pieces have soft or hollow spots where replacement was incomplete. Tap-test a piece before committing: a solid ring is good, a dull thud means voids inside.
Tiger's eye
Mohs 7 and it tumbles well, but its fibrous structure means orientation matters for the chatoyant flash. In a tumbler you have no control over orientation, so you're rolling dice on which stones end up showing the eye.
Chiastolite
Mohs 6.5 to 7.5 — hard enough, but the graphite that forms the natural dark cross is much softer than the surrounding andalusite. Run it too long in coarse grit and the cross undercuts into a groove. Our Chiastolite Fairy Cross Stones are finished with that in mind. If you're wondering how chiastolite differs from staurolite — the other mineral people call a fairy stone — we cover it in fairy stones explained.
Obsidian
Volcanic glass, Mohs 5 to 5.5. It takes a spectacular mirror polish because there's no crystal structure to interfere. But it's glass — it chips conchoidally, and one hard knock puts a crescent divot in an otherwise perfect stone. Run obsidian in its own batch with plenty of plastic pellets for cushioning.
Tier 3: Experienced Tumblers Only
These are real stones people genuinely tumble. They just require modified process, and if you try them on batch two you will be disappointed.
- Fluorite (Mohs 4). Beautiful color zoning, and it has perfect cleavage in four directions, which means it splits along internal planes under impact. Needs short cycles, heavy pellet cushioning, and gentle handling. Our Rainbow Fluorite Tumbled Stones show what it looks like when it works.
- Malachite (Mohs 3.5–4). Soft, banded copper carbonate. Undercuts badly between bands of different density, and it will fizz in acid. Usually finished with a burnishing step rather than a standard polish. See our Genuine Malachite Tumbled Gemstones.
- Calcite (Mohs 3). Very soft, three-direction cleavage, water-reactive over long cycles. Most people who love calcite buy it carved rather than tumbling it — the Blue Calcite Eggs from Peru are hand-shaped for exactly that reason.
- Labradorite (Mohs 6–6.5). Two cleavage directions and the flash depends on how the surface intersects the internal layers. Tumbles acceptably, but hand-shaped labradorite palm stones show far more labradorescence because someone chose the orientation deliberately.
Stones to Keep Out of the Barrel
- Selenite (Mohs 2). Water-soluble gypsum. It will literally dissolve. Never wash it, let alone tumble it.
- Turquoise. Porous and often stabilized with resin. The grit embeds in the pores and the resin degrades.
- Anything with visible cracks. A fracture you can see is a fracture that will open. Break it along the crack first and tumble the sound pieces.
- Sandstone, shale, limestone, most sedimentary rock. Too soft and too porous. It disintegrates into slurry and contaminates the batch.
- Anything you can scratch with a steel nail. Under Mohs 5.5 and it's a specialist project, not a beginner batch.
How to Build a Batch That Actually Works
- Sort by hardness first, appearance second. The prettiest mixed handful is usually the worst batch.
- Mix sizes deliberately. A range from roughly 1/2 inch to 1 1/2 inches works best. Small stones act as a natural grinding medium against larger ones, and a barrel of uniformly large rocks just clunks around doing very little.
- Fill the barrel 2/3 to 3/4 full. Underfilled barrels let rocks tumble too far and hit too hard. Top up with plastic pellets rather than adding rocks you don't want.
- Cull after the coarse stage. This is the single highest-value habit in the hobby. Any stone that's cracked, pitted, or clearly not shaping up gets removed before it wastes three more weeks of your medium, pre-polish and polish grit.
- Never carry grit forward. Rinse the stones, rinse the barrel, rinse your hands. One coarse particle in the polish stage ruins the entire batch, and you'll only discover it at the very end.
The full four-stage process is in our complete guide to tumbling rocks if you want the timings and grit sequence.
Where to Get Good Rough
Finding your own is the best part of the hobby — our beginner's guide to rockhounding covers how to start. But there's a reason experienced tumblers also buy known material: when you're learning the process, you want to eliminate the variable of whether the rock was ever going to work.
A mixed pound from our Tumbled Rocks, Agates & Natural Gemstones collection shows you exactly what a correct finish looks like — genuinely useful as a reference when you're judging your own results. Sold by the piece or the pound.
Load agate. Load jasper. Load quartz. Save the fluorite for batch ten.
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