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Agate is the stone most rock tumbler instructions are written for. It's about 7 on the Mohs scale, dense, colorful and durable, and it takes a bright, glassy polish. Run it through the standard four stages, give the coarse stage at least one to two weeks, keep the barrel properly filled, and you'll get excellent results.
The catches are specific to agate: hollow or crystal-lined centers, delicate bands and "eyes" just under the skin, and a tendency to look finished before it really is. This guide covers how to tumble agate step by step, with stage times from Lortone, Harbor Freight and Kingsley North.
What Agate Is (and Why It Tumbles So Well)
Agate is a variety of chalcedony, microcrystalline quartz with crystals too small to see. Rocktumbler.com describes agate as translucent to semitransparent and usually banded, while jasper is the opaque variety of the same material. Both are about Mohs 7.
Agate forms when silica-rich water deposits layer after layer inside cavities in volcanic rocks such as basalt and rhyolite. The host rock weathers away more easily, so agate nodules collect in streams, fields and beaches. That's why agate is both common and a favorite of rockhounds.
Agate Tumbling Schedule
Agate's Mohs 7 hardness is exactly what the manufacturer schedules assume.
| Stage | Grit | Time for agate | Source | Done when… |
|---|---|---|---|---|
| 1. Coarse | 60/90 silicon carbide | 7 to 10 days; often 14 or more | Harbor Freight, Kingsley North; Rocktumbler.com runs 2+ weeks | Fully rounded, no pits, no rind |
| 2. Medium | 120/220 or 150/220 | About 7 days | Kingsley North, Harbor Freight | Even, smooth matte all over |
| 3. Pre-polish | 500 to 600 | About 7 days | Kingsley North, Harbor Freight | Faint sheen when dry |
| 4. Polish | Aluminum oxide or cerium oxide | 5 to 7 days | Kingsley North, Harbor Freight | Looks the same wet and dry |
| 5. Burnish (optional) | Powdered soap | At least 4 days | Kingsley North, Harbor Freight | Clean, bright surface |
Lortone's simpler rule also fits agate: check after 2 to 3 days, and expect each stage to end after 5 to 7 days. Our rock tumbling timeline calculator adjusts for your starting shape.
How to Tumble Agate, Step by Step
1. Sort and Inspect the Rough
Wash the agate and look closely. Cull or break pieces with:
- Open cracks, which break apart and shed sharp fragments.
- Hollow centers or open cavities, which trap coarse grit and release it during the polish. Rocktumbler.com's rule is to keep any rock with visible voids out of the barrel.
- Chalky or crumbly husk you can rub off with a fingernail.
Group agate with other Mohs 7 stones only: jasper, petrified wood, chert and quartz-family rough. Our guide to the best rough stones for tumbling covers good agate sources.
2. Load the Barrel
Fill the barrel about half to two-thirds full, per Lortone. Mix sizes; Lortone recommends assorted sizes because small pieces fill gaps and add grinding contact. For a 3 lb barrel, Rocktumbler.com suggests rough from about 1/4 to 1-1/2 inches, and Lortone rates its 3A for stones up to 1-1/4 inches.
Rocktumbler.com says durable stones such as agate need little or no media in coarse, at most about 10%. Add more only if you're short on rock.
3. Coarse: Give It Time
Dose 60/90 silicon carbide for your barrel: 4 level tablespoons in a 3 lb barrel per Lortone's chart, or 4 tablespoons per 3 lb drum per Harbor Freight's manual. Add water to just below the top layer of stones, seal, and vent early. Harbor Freight vents at 12 and 24 hours.
Run 7 days, then inspect. If any stone still has pits, flat broken faces or rind, run another week with fresh grit. Kingsley North calls coarse the most important step. With agate, a second coarse run is normal, not a failure.
4. Medium and Pre-Polish
Clean everything thoroughly, then run 120/220 or 150/220 for about a week, and 500 to 600 pre-polish for about a week. Lortone uses aluminum oxide for pre-polish; Harbor Freight and Kingsley North use 600 grit. Top up the barrel with ceramic media as the stones shrink.
5. Polish
Agate responds well to both of the classic quartz polishes:
- Aluminum oxide, which Lortone's sequence uses and most kits include.
- Cerium oxide, widely used for quartz-family stones and glass.
Run 5 to 7 days. Then dry a few stones. If they only shine wet, go back to pre-polish. Our upcoming guide to rock polishing compounds compares the options.
6. Burnish (Optional)
Kingsley North and Harbor Freight both suggest a soap burnish: about 1 tablespoon of powdered soap in a 3 lb barrel, with plastic pellets and water, for at least 4 days. Kingsley North warns against liquid detergents and products with bleach.
Agate Problems and How to Avoid Them
Crystal Centers That Bruise
Many agates have centers of crystalline quartz. Rocktumbler.com lists crystalline quartz among materials that bruise easily and recommends cushioning. If your agate has visible crystal centers, add small ceramic media, up to 50% for very bruise-prone loads. See our guide to ceramic media for rock tumbling.
Hollow and Vuggy Agates
Agates that formed in partly filled cavities may be hollow or have open pockets. They trap grit, and that grit scratches the whole batch in the polish stage. Cut or break them open, or set them aside for display.
Grinding Through the Best Part
Rocktumbler.com notes that the "eyes" in eye agates usually sit just below the outer husk. Banding near the surface can be thin too. A very long coarse run can grind through the feature you liked. With special pieces, check every few days during coarse and move on as soon as the stone is rounded and pit-free.
Dyed Agate
Brightly colored agate is often dyed. Rocktumbler.com lists agate among commonly dyed materials, and dye tends to collect in cracks and in the more porous bands. Tumbling can wear through a surface dye. Natural agate is usually in grays, whites, browns, reds and oranges.
Mixed Batches
Agate is often sold in mixes. Rocktumbler.com's hardness example is agate (7) with obsidian (5 to 6): the obsidian is ground tiny before the agate is shaped. Tumble agate only with similar-hardness stones.
Beach and Stream Agate
Found agate is often the best agate. Lake Superior agates, for example, collect on Great Lakes shores, and Rocktumbler.com notes they're Minnesota's state gemstone. Rocktumbler.com's before-and-after page describes unpolished Lake Superior nodules turning "super-bright," with their banding showing.
Don't skip coarse because a beach agate looks smooth. Rocktumbler.com says beach and stream stones carry a weathering rind, typically 1/8 to 1/4 inch, and impact bruises. In its own test, one week of coarse grit on banded beach agate wasn't enough: fractures showed after polishing, and the batch went back to coarse. Plan on one to two weeks.
Always check local rules before collecting; many parks and beaches don't allow it.
Rotary or Vibratory for Agate?
Start in a rotary tumbler. Agate rough needs real shaping, and vibratory tumblers do little rounding. Many hobbyists then finish in a vibratory, where Rocktumbler.com estimates one to three days per step instead of a week. Our rotary vs vibratory guide explains the workflow. For machines, see our best rock tumbler picks, and for the general process, our how to rock tumble guide.
FAQ
How long does it take to tumble agate?
About four to six weeks in a rotary tumbler, and often longer. Agate is about Mohs 7, so it follows the standard manufacturer schedules: 7 to 10 days of coarse grit per Harbor Freight's manual, then about a week each of medium, pre-polish and polish. Rocktumbler.com says its own coarse stage usually runs at least two weeks.
What grit do you use for agate?
The standard four-stage sequence: 60/90 silicon carbide for coarse, 120/220 or 150/220 for medium, a 500 to 600 pre-polish, then an aluminum oxide or cerium oxide polish. Lortone's manuals use aluminum oxide for both pre-polish and polish.
Why are my tumbled agates not shiny?
The most common causes are a coarse stage that was too short, grit trapped in cracks or hollow centers and released during polishing, or crystalline quartz centers that bruised. Return the stones to coarse if pits remain, cull hollow pieces, and add ceramic media to cushion the batch.
Can I tumble agate with other rocks?
Yes, with stones of similar hardness. Jasper, petrified wood, chert and other Mohs 7 materials tumble well with agate. Keep softer stones such as obsidian or sodalite in a separate batch, or they'll be ground away before the agate is shaped.
Can I tumble beach agates?
Yes, and they're some of the best rough you can find. Don't skip coarse grit, though. Rocktumbler.com says beach and stream stones carry a weathered rind and impact bruises, and that one week of coarse wasn't enough for its banded beach agates; plan on one to two weeks.