Checked against tumbler manufacturers' manuals · Sep 2026

Rock Tumbling Calculator & Beginner Guide

Work out the grit, time and amounts for your tumbler and your rock, check whether a rock is worth tumbling at all, and get your first batch right, whether you bought a tumbler or unwrapped one.

The short answer

A rotary tumbler uses four steps: 60/90 coarse grit, 150/220 medium, 500 pre-polish, then an aluminum oxide polish, about 7 days each. Use about 2 level tablespoons of grit per pound of rock, fill the barrel about two-thirds full and add water to just below the top of the rocks. Expect 4–6 weeks in total, or longer if the coarse step needs extra weeks.

Can I Tumble This Rock?

Type the rock's name to check its hardness and any reason not to tumble it. Not sure what it is? Identify it from a photo first.

Tumbling Grit & Time Calculator

Pick your tumbler, how hard your rock is and how much you are tumbling. The schedule updates as you go.

Tumbler type

Your rock right now

Type of stone

Pounds of rock in the barrel

1. Coarse grind
Grit
60/90 silicon carbide
Time
7 days, often a second week
Amount
4 tbsp
Done when every stone is rounded with no sharp edges. If not, rinse and run another week with fresh grit; tough agate can take 3–4 weeks.
2. Medium grind
Grit
150/220 silicon carbide
Time
7 days
Amount
4 tbsp
Done when a dry stone has an even, smooth, frosted surface.
3. Fine grind (pre-polish)
Grit
500 grit (silicon carbide or aluminum oxide pre-polish)
Time
7 days
Amount
4 tbsp
Done when the stones feel very smooth and some show a slight luster.
4. Polish
Grit
Aluminum oxide polish (or cerium oxide)
Time
7 days
Amount
4 tbsp
Done when the stones look the same dry as wet. A clean barrel matters most here.

Total: About 4–6 weeks (up to 8 for tough rough)

Media: 0–10% ceramic media in the coarse step, then top up to 2/3 full as the rock wears down.

Every step: fill the barrel about 2/3 full and add water to just below the top of the rocks.

Tumble one type of rock, or rocks of the same hardness, per batch. Harder stones grind softer ones down.
Never pour used grit slurry down a drain: it settles and sets like concrete. Rinse over a bucket and put the settled mud in the trash.

Best (and Worst) Rocks for Tumbling

Rocks between Mohs 6 and 8 work best. Beginners should start with agate, jasper or petrified wood.

Rock	Mohs	Verdict	Why
Jasper	· Mohs 7	Great	The classic first-batch stone: durable and takes a very bright polish. An extra week in coarse grit improves the shape.
Agate	· Mohs 7	Great	Great for beginners. Beach and river agates can need several weeks of coarse grit to remove bruised rinds.
Petrified wood	· Mohs 7	Great	Silicified wood tumbles like jasper. Pick pieces without voids or cracks.
Chert and flint	· Mohs 7	Great	Tough and slow to round; two weeks of coarse grit gives better shapes.
Carnelian, bloodstone, chrysoprase	· Mohs 7	Great	Varieties of chalcedony that follow the standard schedule.
Amethyst, rose quartz, clear quartz	· Mohs 7	With care	Crystal quartz bruises at the edges. Plan 2–4 weeks of coarse grit and lots of ceramic media.
Tiger's eye	· Mohs 7	With care	Bruises like crystal quartz. Keep the slurry wet; don't let it dry to dust.
Unakite, sodalite, rhodonite	· Mohs 5.5–7	With care	Shape quickly and some split along cleavage planes. Use at least 25% ceramic media.
Labradorite, amazonite, moonstone	· Mohs 6–6.5	With care	Feldspars with cleavage: they chip if tossed hard. Fill the barrel properly and cushion them.
Obsidian and Apache tears	· Mohs 5–5.5	With care	Skip the coarse grit, use at least 50% media and vent the gas they produce.
Glass and goldstone	· Mohs about 5–6	With care	Skip coarse grit and vent gas daily. Fast toy tumblers can bruise or crack glass.
Turquoise and opal	· Mohs 5–6.5	Avoid	Turquoise is porous and carries grit between steps; opal is brittle and crazes.
Garnet	· Mohs 6.5–7.5	Avoid	Soft inclusions wear into pits. Most tumbled garnet disappoints.
Pyrite and marcasite	· Mohs 6–6.5	Avoid	Brittle, and metallic minerals give off gas.
Fluorite, calcite, malachite	· Mohs 3–4	Avoid	Too soft for a standard batch; malachite dust is also toxic.
Halite, selenite, gypsum	· Mohs 1.5–2.5	Avoid	They dissolve or break down in water. Never tumble them.

Got a Rock Tumbler as a Gift? Your First Batch

Based on the National Geographic Hobby tumbler's manual; most beginner kits work the same way. Kits come with about ½–1 lb of mixed rough and four numbered grits.

1
Fill the barrel about half to two-thirds full of rock. On a National Geographic kit, overfilling trips a switch that turns the motor off.
2
Add grit #1 (coarse) and water until it just reaches the top of the rocks. Seal the lid and wipe any grit off the rim first, or it will leak.
3
Run grit #1 for the kit's 3–5 days, then rinse a few test stones. Hard stones usually need longer: keep going, with fresh grit each week, until no sharp edges are left.
4
Rinse everything over a strainer and bucket, not the sink. Clean the stones, barrel and lid rim until no grit is left.
5
Run grit #2 (medium) for 6–8 days, grit #3 (pre-polish) for 6–8 days and grit #4 (polish) for 7–10 days, rinsing thoroughly between each.
6
Newer kits include GemFoam: run it for 3 days at the end to brighten the shine.
7
The Hobby tumbler's timer runs up to 7 days, so re-arm it for the 8- to 10-day steps.

Tip: kit stones such as tiger's eye and rose quartz are crystal quartz, which bruises easily. If they come out with white marks on the edges, give the next batch more time in coarse grit and a fuller barrel.

10 Beginner Mistakes

Pouring grit slurry down a drain. It settles and hardens like concrete.
Leaving grit behind between steps. One grain of coarse grit in the polish scratches the whole batch.
Moving on before the coarse step is finished. Later grits won't fix a bad shape.
Mixing rock types or hardnesses in one batch.
Tumbling rocks with cracks, holes or gritty, crumbly surfaces.
Filling the barrel too little (stones chip and bruise) or too much (nothing moves).
Adding too much water, which cushions the rocks so the grit can't work.
Using sand as grit. Sand is quartz, the same hardness as agate, so it barely cuts.
Putting a box over a noisy tumbler. The motor overheats.
Ignoring gas from obsidian, glass or metallic minerals until the lid pops off.

Troubleshooting

Smooth but dull or hazy after polish

Burnish: tumble for about an hour with water and about ½ tablespoon of grated plain bar soap per pound of rock.

Shiny when wet, dull when dry

The pre-polish or polish isn't finished. Run it longer until the stones look the same dry as wet.

Scratches after polishing

Grit carried over, or a crumbly rock shed grains. Remove bad stones, go back to the medium grit for a few days, then repeat pre-polish and polish.

White bruises or chips on edges

The barrel is too empty or too fast. Fill to about 2/3 and add small ceramic media.

Rocks aren't rounding

The barrel is too full, the grit is worn out or the rocks are very hard. Remove some rock, add fresh grit and give it more weeks.

Barrel bulging or leaking

Gas pressure or grit on the seal. Vent the barrel outdoors or over a tub, clean the rim and reseal without over-tightening.

What Tumbling Costs

Grit for a first batch in a small rotary barrel costs roughly $7–$15, or about $3–$5 per pound of rock. Vibratory tumblers use much less, often under $1 per pound.
Electricity is small: about $2–$4 for a five-week batch in a typical small rotary tumbler.
Expect to lose weight: 10 lb of rough usually ends as 6–7 lb of tumbled stones, and 10–30% of the volume goes in the coarse step alone.
Tumbling is a hobby rather than a way to save money: ready-tumbled stones can cost as little as $12–$14 per pound.

Rock Tumbling Questions

Get answers to common questions about rock identification, rockhounding, and using our platform.

In a rotary tumbler the standard four steps take about a week each, so plan on 4–6 weeks, and up to 8 for tough rough that needs extra weeks in coarse grit. A vibratory tumbler finishes pre-shaped rough in about 1–2 weeks, but it can't round sharp rough.

The usual rotary sequence is 60/90 silicon carbide (coarse), 150/220 or 120/220 silicon carbide (medium), 500 grit silicon carbide or aluminum oxide (pre-polish), then an aluminum oxide polish. Cerium oxide is a common alternative polish, especially for feldspars and obsidian.

About 2 level tablespoons of grit per pound of rock (and ceramic media) in each rotary step. Manufacturers' charts work out slightly lower, about 1–2 tablespoons per pound. Vibratory tumblers use about half a tablespoon per pound.

Fill a rotary barrel about two-thirds full of rock (sources range from half to three-quarters), then add water to just below the top of the rocks. Too little rock makes stones chip; too much stops them tumbling.

Yes, if they are hard (Mohs 6–8), solid and not crumbly. Agate, jasper, chert and quartz pebbles work well. Skip weathered, cracked, porous or gritty rocks, and rocks that crumble when you rub two together. Use the checker above to look up a rock's hardness.

Usually the pre-polish or polish step wasn't finished, grit carried over from an earlier step, or a film from the polish is left on the stones. Run the step longer, clean everything thoroughly between steps, or burnish with soap for about an hour.

No. Every tumbler manufacturer warns against it: the slurry settles in pipes and hardens like concrete. Rinse over a bucket, let the mud settle, and put the solids in the trash.

Anything that dissolves in water (halite, selenite, gypsum), toxic minerals (cinnabar, realgar, orpiment, galena, malachite dust), asbestos-bearing rock, and very soft stones such as calcite and fluorite unless you use an advanced method. Porous stones like turquoise and brittle ones like opal also give poor results.

Sources

Schedules and amounts follow the consensus of tumbler makers' manuals and long-running tumbling guides. Where they disagree we give the common value and the range.