Rock Tumbling Grit: What It Is and What to Buy
Grit is the part beginners get wrong, and it decides whether a batch comes out glassy or dull. Here is what the four stages do, how much you need, and why sand is not a substitute.
In this guide
Quick route through the page: start with the main takeaway, then use the sections below to go deeper where you need it.
- What Tumbling Grit Actually Is
- The Four Stages
- How Much Grit You Need
Tool review
Use this page to figure out whether the tool deserves pack space, not just to skim a shopping list.
Table of Contents
Grit is the cheapest part of tumbling and the part that decides how a batch turns out. Get the sequence right and ordinary agate comes out glassy. Get it wrong and you run the whole thing again, four weeks lost.
If you have not tumbled before, start with our tumble polishing guide for the full process. This page is about the abrasive itself.
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What Tumbling Grit Actually Is
Tumbling grit is silicon carbide. It is not dug out of the ground. It is made by heating petroleum coke and sand together in an electric resistance furnace, and the result is one of the hardest materials known — 9+ on the Mohs scale.
That number is the whole reason it works. An abrasive has to be harder than what it cuts. Most tumbling rock is agate, jasper or quartz, all sitting at about 7. Silicon carbide is well above them, so it bites.
The last stage is different. Polish is not silicon carbide but aluminium oxide or cerium oxide, and it is not there to cut. It burnishes a surface the earlier stages already made smooth.
The Four Stages
A rotary tumbler runs four stages, and each has a job. RockTumbler.com gives the sequence as coarse 60/90, medium 150/220, fine 500, then a polish such as TXP aluminium oxide.
- Stage 1 — coarse 60/90. Shapes the rock. Knocks off points and edges. This is where a rough stone becomes a round one, and it is the longest stage.
- Stage 2 — medium. Removes the scratches stage 1 left behind. Vendors label this one differently. You will see 150/220, 180/220 and 120/220 on the shelf. Any of them does the job.
- Stage 3 — fine 500. A pre-polish. Stones start to look dull-smooth rather than rough here, which fools people into stopping early.
- Stage 4 — polish. Aluminium oxide or cerium oxide. This is the shine.
Grit chart
| Stage | Material | Grit | What it does |
|---|---|---|---|
| 1 — coarse | Silicon carbide | 60/90 | Shapes the rock and removes edges |
| 2 — medium | Silicon carbide | 150/220 (also sold as 120/220 or 180/220) | Removes coarse scratches |
| 3 — fine | Silicon carbide | 500 | Pre-polish, smooths the surface |
| 4 — polish | Aluminium oxide or cerium oxide | varies by brand | Brings up the shine |
How Much Grit You Need
The working rule for rotary tumblers is about two ounces of silicon carbide per pound of rock. That is roughly two tablespoons. A 3 lb barrel therefore takes about six ounces per stage.
Multiply that across four stages and one full 3 lb run uses close to a pound and a half of material. That number is worth knowing before you buy, because it tells you how fast a small kit disappears.
Buy a Kit, or Buy Stages?
For a first batch, buy a kit. It gives you the right amount of each stage in the right order, with no quantities to work out. The National Geographic refill is the one most beginners end up with, and it covers all four stages.
Check the National Geographic Rock Tumbler Grit and Polish Refill Kit on Amazon
If your tumbler is a 3 lb rotary and you also need filler media, the MJR kit bundles plastic pellets with the four stages. Filler matters more than beginners expect. It cushions the load and keeps smaller stones from bruising each other.
Check the MJR Tumblers grit kit refill with 12,000 polish and plastic pellets on Amazon
Buying stages separately
Once you tumble regularly, kits stop making sense. You burn through coarse far faster than polish, so buying stages on their own is cheaper and wastes less.
Check the Fundamental Rockhound coarse 60/90 silicon carbide (2 lb) on Amazon
Check the Polly Plastics medium 180/220 silicon carbide, stage 2 (2 pack) on Amazon
Lapidary suppliers such as Kingsley North and Polly Plastics also sell grit by the pound direct. That is usually the cheapest route of all once you run batches back to back.
Why Sand Does Not Work
This comes up constantly, and the answer is simple hardness. Sand is mostly quartz, and quartz is 7 on the Mohs scale. Agate, jasper and chalcedony — the stones people actually tumble — are also about 7.
Nothing can grind away a material as hard as itself. Sand in a barrel just rounds itself off while your rocks sit there unchanged. That is the difference silicon carbide at 9+ makes.
To check the hardness of what you are about to tumble, our Mohs hardness field guide covers how to test it.
Mistakes That Waste a Batch
- Grit carried between stages. One coarse grain left in the barrel will scratch through a whole polish run. Wash the barrel, the lid, the seal and the stones between every stage.
- Mixing hardnesses. Tumble stones of similar hardness together. A soft stone in with agate breaks down and muddies the grit.
- Too little rock in the barrel. RockTumbler puts a rotary barrel at half to two-thirds full. An under-filled barrel lets stones drop instead of slide, and that chips them.
- Pouring grit down the drain. Spent grit and rock mud set like cement in a trap. Let it settle in a bucket, pour off the water, and bin the sludge.
Ready to run a batch? Our tumble polishing guide walks through the full sequence, and our tumbler guide covers the machines themselves.
Frequently Asked Questions
It is silicon carbide, and it is man-made. RockTumbler.com describes it as a manufactured abrasive, sold in granules sized for shaping and smoothing rock. It is produced by heating petroleum coke and sand together in an electric resistance furnace. The result is one of the hardest materials there is, at 9+ on the Mohs scale. That hardness is the whole point. The abrasive has to be harder than the rock, or nothing happens.
There is no single best grit, because you use four in sequence. Coarse 60/90 silicon carbide shapes the rock. Medium comes next, usually sold as 150/220. Fine 500 smooths it. Then a polish, most often aluminium oxide, brings up the shine. Skipping a stage does not save time. It leaves scratches the next stage is too fine to remove, so you end up running the batch again.
Silicon carbide, and in practice nothing else. People try sand, and it does not work. Most sand is quartz, which sits at 7 on the Mohs scale, and so does the agate and jasper that most people tumble. An abrasive cannot cut something as hard as itself. Silicon carbide is 9+, which is why it does. For the final stage you swap abrasive for polish, usually aluminium oxide or cerium oxide.
Not usefully. Without an abrasive the rocks only knock against each other, which chips edges instead of grinding them down, and nothing gets smooth. There is one narrow exception: a final burnishing run with soap and plastic pellets, after polish, to clean the stones. That is a finishing step, not a substitute for the four stages.
Yes, and it wastes money without working faster. The abrasive has to sit between the stones to cut, and a barrel packed with grit turns into a paste that cushions them instead. Stick to about two ounces per pound of rock. Adding more does not shorten a stage — only running the stage longer does that.
Lapidary suppliers sell it by the pound, and that is the cheapest route once you tumble regularly. Kingsley North and Polly Plastics both stock the full range. For a first batch a four-stage refill kit is simpler, because it gives you the right amount of each stage without guessing. Buy single stages later, when you know which one you run out of first.
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Sources & References
- What is Rock Tumbler Grit? — RockTumbler.com
- Silicon Carbide Grit — Rock Tumbling Supplies — Kingsley North
- Rock Tumbling Grit Kit — Polly Plastics
Sarah Mitchell
Field Editor, The Rockhounding Hub
Sarah has been collecting rocks and minerals for over 15 years across the western US. She specializes in agate hunting and beginner education.
