TOOLS How I tumble sand polymer clay - river rocks

I've decided to update this discussion, since I've collected a bit more information. For those who know me, more info than you'll ever need. ;-) Due to the volume, I've broken up this discussion into several pages.

Hand sanding is still the best method towards the best finish, but it comes at a high price, especially if you have a lot of beads to sand. I've talked to many folks who make polymer clay beads who accummulated a small ton of unfinished beads, beads waiting to be sanded... and waiting and waiting. I used to be in that crowd.

Using a rotary or vibrating tumbler is a great way to process a batch of beads/pieces at a time. You can process polymer clay in a matter of hours, especially if you smooth your pieces before baking. I used to make my own sandpaper chips; tumbling my beads in a soup of chips and water.

Recently I've explored using a different tumbling media: small rounded smooth rocks - river rocks.

It's now my method of choice, far and away much preferred over using sandpaper chips. In combination with using a vibrating tumbler, I can check the progress at anytime without stopping the machine. Fret not, the stones will also work in rotary tumblers.


I bought a sack of 3/8" river rock from a local rockery for about $16. Damn thing must have weighed 100 pounds! Compared to the other kinds of rocks, these were about 4 times the cost. Apparently pretty rounded rocks are more expensive.

River rock is composed of various types of rocks that are generally rounded, having been naturally tumbled in a river bed.

Rockeries near me sell what they call PAMI, river rock pebbles from western Montana. They're also call rainbow river rocks. As you can tell from the picture, there's a good reason for that name. They're naturally and beautifully colorful, once they're rinsed.


First sort

I sorted them by hand; small batches at a time; culling the roundest pebbles; discarding anything that was jagged, cracked, rough. Yes, it is a mind numbingly tedious process, examining each pebble by sight, touch and/or feel. But such was my determination.

If you know a rock hound ask for great nicely tumbled (but not polished) rocks. That may save you from the mind numbing hand sorting.


The first round of culled pebbels looked very sweet. The site of naturally gently rounded shapes of various sizes and earth tone colors is soothing, in a way. :)

When I had more than enough to properly fill my vibrating tumbler, I tossed in the pebbles and a batch of unsanded lentil beads and turned the machine on. There was quite a racket because rocks are banging against the plastic tub. The the unique motion created by the vibrating tumbler is on track.

I slowly poured in a little bit of water, noting that too much water will suppress the action. The water immediately dampened the noise. I added just enough water so that everything was wet. Tumble time: 6 hours.

It worked! I kept the machine running while it slowly cycled the beads to the top, where I pluck them out of the bowl. Then I stop the tumbler and pour the rock 'soup' into a colander and rinse the rocks, separately rinse the beads.

I'd estimate, because of the coarsest rocks, the beads had a sanded smoothness equivalent to CAMI 400 - 500 grit.

I've called this set of rocks - stage 1.


Second sort

I revisited my batch of river rocks, the rounded ones, and culled out the smoothest ones from the not as smoothest. It wasn't too hard to visually distinguish between especially when the rocks are dry. This second culled batch, called stage 2 because of the smoother stones, I expected would abrade at a finer grit level.

The smoother ones didn't look too different from the less smooth ones, but they definitely felt different! :)

I filled my Lortone 3A three quarters full with a mixture of those stage 2 stones, the tumbled lentils and enough water to barely cover the top of the batch. I used my rotary tumbler simply because I didn't have as many of the really smooth rocks. Tumble time: 4-5 hours.

It worked beautifully. I'd estimate the beads had a sanded smoothness equivalent to CAMI 700-800 grit. They buffed beautifully.

I tried rotary tumbling with highly polished stones. I used my rotary tumbler simply because highly polished stones tend to be semi-precious and expensive. I only purchased a pound of these stones. They didn't seem to work too well. Perhaps being so polished, they've not got sufficient teeth to abrade anything.

Advantages of tumbling with river rocks

What I like the best about all this:

  • River rocks aren't expensive.
  • River rocks are large enough that they're easy to sort from polymer clay beads.
  • Unlike sandpaper, river rocks as a tumbling media will last fovever.
  • River rocks are pretty when wet.
  • Vibrating tumblers are cool because they don't need to be sealed up or lidded. You can easily check the tumbling progress without stopping and unsealing the barrel.
  • I can finally use that big ole vibrating tumbler which until recently, had been collecting dust for the past 4-5 years.

I'm still experiementing with rock as tumbling media, so check back once in a while.

For those of you who don't have a local rockery or rock source, there are plenty online sources for rocks for all manner of purpose.

Here's one:

Make sure to look for the smallest possible size, in the 1/4" - 3/8" range and look for smooth but unpolished ones. Note, they don't have to be rainbow river rocks, just small rocks (pebbles) that are round and smooth.

If you plan to make your own sanding stones by tumbling them with grit, here are a few sites that may be useful:

About My Rainbow River Rocks

Finally, after much research, a couple of helpful landscaping store employees and several very knowledgable members of a local gem and geology society, I found out what these rocks are considered. Collectively, they bandied about terms like:

  • PAMI
  • pixie pebbles
  • rainbow river rocks
  • western Montana river rocks
  • chert
  • quartz
  • jasper
  • flint
  • agates
  • mudstone
  • argillite
    • A metamorphic rock, intermediate between shale and slate
    • A rock containing chiefly clay materials; derived from claystone, siltstone, or shale; used locally as building stone, although rarely produced commercially.
    • A fine-grained formerly sedimentary rock that morphed; composed predominantly of indurated clay particles. Argillites are basically lithified muds and oozes. They contain variable amounts of silt-sized particles. The argillites grade into shale when the fissile layering typical of shale is developed. Another name for poorly lithified argillites is mudstone. These rocks, although variable in composition, are typically high in aluminium and silica with variable alkali and alkaline earth cations. The term pelitic or pelite is often applied to these sediments and rocks. Metamorphism of argillites produces slate, phyllite, and pelitic schist.

That's what I get for asking! :D

OK, so the PAMI (a.k.a. 'pixie pebbles' and 'rainbow river rocks') I got at the local rockery is a collection of several different types of rock from northwestern Montana. The most noteworthy characteristic are the color variations. Any random collection is simply beautiful, hence the name "Rainbow River Rocks". I shall call mine "Rainbow Sanding Stones" since I've put my colorful collection of rocks to work sanding my polymer clay pieces.

The predominant river rock from this region is, argillite, a clay rock. Thus I'm using natural clay to sand polymer clay. For some reason, this seems elegantly appropriate... or amusing... or both.

Suh-weet! :)

There's nothing better than having a great tool when you need it.


Last update to this page: 21 Oct 07. Send comments, questions or suggestions to Desiree McCrorey.