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The best rocks for tumbling are hard, dense and free of cracks: jasper, agate and petrified wood top every list, with chert, flint and many quartz-family stones close behind. All of them sit around 7 on the Mohs scale, which is exactly the hardness that standard tumbler instructions are written for.
Most rocks, though, are poor candidates. Rocktumbler.com, a long-running hobby retailer, puts it bluntly: most types of rock do not perform well in a tumbler. This guide shows you which rocks for tumbling work, which need special handling, and which to leave on the beach.
The Four Tests Every Tumbling Rock Must Pass
Before hardness even comes up, a rock has to survive a month of grinding without falling apart. Rocktumbler.com uses four criteria, and they're the quickest way to sort a bucket of finds.
- Hardness. Aim for about 6 to 8 on the Mohs scale. Rocktumbler.com says stones above 8 cut very slowly and often polish poorly, and soft stones are hard to polish at all.
- Density. No pores, voids or holes. Pores trap coarse grit and release it during the polish stage, where it scratches every stone in the barrel.
- No fractures. Cracked rock breaks apart in the barrel, and the sharp fragments chip and scratch the rest of the load. Break fractured pieces along the crack first, or discard them.
- Smooth texture (the rub test). Rub two rocks together firmly. If grains come off, the rock will shed grit in the tumbler and won't polish.
A rock that fails any one test can ruin a whole batch, not just itself. Be ruthless.
Best Rocks for Tumbling: The Beginner Shortlist
These are the good rocks to tumble on your first runs. They sit at about Mohs 7, per Rocktumbler.com's hardness table, and tolerate the standard schedule.
| Stone | Mohs | Why it works | Watch out for |
|---|---|---|---|
| Jasper | 7 | Very durable, opaque, bright polish | Poorly silicified pieces that shed grit |
| Agate | 7 | Colorful bands, inexpensive, polishes brightly | Hollow or crystal-lined nodules |
| Petrified wood | 7 | Largely the same material as jasper | Voids and cracks along the grain |
| Chert and flint | 7 | Common in streams and fields, varieties of jasper | Chalky outer rind |
| Aventurine | 7 | Sparkly green quartz, takes a good shine | Crystalline quartz can bruise |
| Tiger's-eye | 7 | Chatoyant bands show well when polished | Bruises without cushioning |
| Mookaite | 7 | Vivid reds and yellows | Soft, chalky patches in cheap rough |
Jasper is Rocktumbler.com's top recommendation for a first batch, followed by agate and petrified wood. If you buy rough rather than collect it, a bag of agate, jasper and petrified wood is the safest place to start. Our guide to the best rough stones for tumbling compares bulk bags.
Rocks That Work With Extra Care (Mohs 5 to 7)
These stones can tumble beautifully, but they need their own batch, shorter coarse runs or cushioning. Hardness values are from Rocktumbler.com's table.
| Stone | Mohs | The catch |
|---|---|---|
| Unakite, granite, basalt | 6 to 7 | Mixed minerals; basalt has a thick weathering rind |
| Amazonite, labradorite | 6 to 7 | Feldspars with cleavage planes that split under impact |
| Moonstone | 6 to 6.5 | Feldspar; same cleavage problem |
| Jadeite, nephrite | 6 to 7 | Tough, but slow to polish |
| Rhodonite | 5.5 to 6.5 | Softer; shorten coarse |
| Opal | 5.5 to 6.5 | Fragile; best left to experienced tumblers |
| Sodalite | 5.5 to 6 | Grinds down fast in coarse grit |
| Obsidian, Apache tears | 5 to 6 | Brittle; chips and bruises easily |
| Turquoise, lapis lazuli | 5 to 6 | Soft; often stabilized or dyed |
Two handling rules cover most of this group:
- Cushion fragile stones. Rocktumbler.com recommends ceramic media or plastic pellets for obsidian and feldspars, and says it uses at least 50% media in the barrel for especially fragile rough. Our guide to ceramic media for rock tumbling covers sizes and amounts.
- Shorten the coarse stage. For obsidian and Apache tears, Rocktumbler.com skips coarse grit entirely and starts in medium.
Mohs hardness scale for rock tumbling
Select a step on the scale, or a stone below, to see which tumbling stones sit there and what that hardness means in the barrel.
Reference minerals: the standard Mohs scale. Tumbling stone hardness values and tumbling notes: Rocktumbler.com's hardness table and tumbling guides.
Rocks to Avoid in a Rock Tumbler
Some stones are a waste of grit no matter how careful you are. Rocktumbler.com's troubleshooting page and its hardness guide flag these.
Too soft (below about Mohs 5). Calcite and marble (3), fluorite (4), malachite and rhodochrosite (3.5 to 4) and serpentine (3 to 5) lose a lot of size in coarse grit and bruise easily. They can be tumbled in a separate, closely watched batch, but they're poor choices for beginners.
Grainy or poorly bound rock. Sandstone and incompletely silicified jasper shed grains that act like stray coarse grit. Rocktumbler.com says very few sedimentary rocks tumble well.
Foliated metamorphic rock. Most gneiss and schist, and quartzite that isn't highly metamorphosed, break apart along their layers.
Weathered rock. Rocktumbler.com says weathered stones always perform poorly, because their grains come loose.
Inclusion-heavy gem rough. Garnet often pits because of mineral inclusions. Rocktumbler.com reports that rough sold as ruby or sapphire (corundum) has been consistently unpolishable in its experience.
Anything with holes or cracks. Voids trap grit, and fractures shed sharp fragments.
Jewelry with set stones. Tumbling loosens settings and can damage the metal. Tumble loose stones only.
Dyed stones. Howlite and magnesite are often dyed to imitate turquoise. They tumble, but Rocktumbler.com notes that many people find them uninteresting once the surface dye wears.
Why Hardness Decides Your Timeline
Grit wears stone down by scratching it, so softer stones lose material much faster. That's why the standard "one week per stage" schedule only applies to stones around Mohs 7.
The manufacturer manuals all assume that hardness:
- Lortone says to check after 2 to 3 days and expect each stage to finish in 5 to 7 days.
- Harbor Freight's manual for its 6 lb dual-drum tumbler calls for 7 to 10 days of 60-90 coarse grit, about 7 days of 220, a week of 600 and 5 to 7 days of polish.
- Kingsley North's rotary instructions call the coarse stage the most important step and say to repeat it with fresh grit if stones aren't evenly rounded.
For softer stones, Rocktumbler.com suggests cutting about 1-1/2 days of coarse time for every Mohs point below 7. Here's what that looks like as a starting point:
| Hardness group | Example stones | Coarse stage starting point | Medium, pre-polish, polish |
|---|---|---|---|
| 7 | Agate, jasper, petrified wood | 7 days minimum, often 14 or more | About a week each, per Lortone, Harbor Freight and Kingsley North |
| 6 to 7 | Unakite, amazonite, labradorite | About 5.5 to 7 days | About a week each; check often |
| 5 to 6 | Sodalite, obsidian, turquoise | About 4 to 5.5 days, or skip coarse | Check every 2 days |
| Harder than 7 | Beryl, topaz | Longer than 7 days | Rocktumbler.com says polish may also need more than a week |
Our rock tumbling timeline calculator does this math for your stone.
Never Mix Hardness in One Barrel
Rocktumbler.com's example makes the point well. Two pounds of agate (7) and two pounds of obsidian (5 to 6) tumbled together means the obsidian gets ground tiny before the agate is shaped. Tumbled separately, at 7 days and 5 days of coarse, you get larger obsidian, finish in 12 days instead of 14, and save two days of power and wear. Sort first, every time.
Size: What Fits in Your Barrel
Rock size matters almost as much as rock type.
- 3 lb barrels (Lortone 3A, Thumler's A-R1 and A-R2): Rocktumbler.com recommends a mix from about 1/4 inch to 1-1/2 inches. Lortone rates its 3A for stones up to 1-1/4 inches.
- Large barrels (Thumler's A-R12 and Model B): 1-1/2 to 3 inches works well, with small pieces mixed in.
- General ceiling: nothing bigger than about half the barrel's diameter.
Always load a range of sizes. Lortone recommends assorted sizes, because small stones fill the gaps between large ones and add grinding contact points. If you're short on small rough, ceramic media fills the same role.
Beach and Stream Rocks: Yes, but Don't Skip Coarse
Found rocks are free, and many are excellent. Agate, jasper, chert and quartzite pebbles turn up on beaches, in stream beds and in dry washes. Rocktumbler.com notes that agate nodules often weather out of basalt and collect in streams.
The trap is assuming a smooth pebble can start in medium grit. Rocktumbler.com says stream- and beach-rounded stones need as much coarse grinding as angular rough, for two reasons:
- A weathering rind, typically 1/8 to 1/4 inch thick, that has to come off before the stone will polish.
- Impact bruises, tiny curved fractures from waves and currents, which need at least the outer 1/8 inch removed.
In its own tests, one week of coarse grit on banded beach agate wasn't enough; fractures showed up after polishing, and the batch went back to coarse. Plan on one to two weeks of coarse in a small rotary.
Sort beach finds hard, too. From a 40 lb bag of basalt cobbles, Rocktumbler.com kept fewer than half.
Where to Get Good Rough
- Buy bagged rough from lapidary suppliers or reputable sellers. Look for a single material or a hardness-matched mix, a stated size range and photos of the actual rough. Our rough stones for tumbling guide compares options.
- Collect it yourself on public land where collecting is allowed. Check the local rules before you pocket anything, since parks and many beaches prohibit it.
- Try fee-dig sites and rock clubs. Clubs often swap or sell local rough cheaply, and members know which local stones polish.
Once your stones are finished, our guide to tumbled stones helps you identify and use them, and our upcoming agate tumbling guide covers the most popular material in depth.
Common Mistakes When Choosing Rocks
- Mixing hardness. The softer stones disappear or come out pitted.
- Keeping cracked stones "to see what happens." They break and scratch the rest of the load.
- Tumbling porous rock. It stores grit and releases it in the polish.
- Skipping coarse on beach pebbles. You'll polish over bruises and rind and get a dull finish.
- Overloading with big stones. Pieces over half the barrel diameter don't tumble; they slide and crush smaller stones.
- Expecting gem rough to polish. Cheap corundum and heavily included garnet rarely do.
For the step-by-step process, see our how to rock tumble guide.
FAQ
What are the best rocks for tumbling for beginners?
Jasper, agate and petrified wood. All three sit around 7 on the Mohs scale, take a bright polish, and tolerate the standard one-week-per-stage schedule that Lortone, Harbor Freight and Kingsley North publish. Rocktumbler.com ranks jasper first because it is so durable.
Can I tumble rocks I find on the beach?
Often, yes, if they are hard, dense and unfractured. Agate, jasper, chert and quartzite pebbles are common good finds. Skip anything crumbly, porous, layered or cracked, and don't skip the coarse stage: Rocktumbler.com says beach and stream stones carry a weathered rind and impact bruises that coarse grit has to remove.
Can you tumble different rocks together?
Only if they share a similar Mohs hardness. Rocktumbler.com's example is agate (7) with obsidian (5 to 6): by the time the agate is shaped, the obsidian is ground far too small. Sort your rough into hardness groups and run each group separately.
What rocks should not be tumbled?
Soft stones below about Mohs 5 (calcite, marble, fluorite, malachite) for beginners, plus any rock that is porous, fractured, weathered or gritty. Sandstone, most schist and gneiss, and poorly silicified jasper shed grains that scratch the whole batch. Rocktumbler.com also reports that rough sold as ruby or sapphire rarely polishes.
How big should rocks be for a rock tumbler?
For a 3 lb barrel, Rocktumbler.com recommends a mix from about 1/4 inch to 1-1/2 inches, and Lortone rates its 3A for stones up to 1-1/4 inches. Larger barrels such as Thumler's Model B handle 1-1/2 to 3 inches. Always load a mix of sizes.