How to Polish Stones in a Rock Tumbler

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Answers to frequently asked questions about how to polish stones in a rock tumbler.

Rocks are commonly polished in a rock tumbler. The basic procedure is to tumble the rocks with progressively finer grits and polishes until the desired shape and shine is achieved. It may take 4 to 6 weeks to finish a batch. I use the following procedure with my rotary tumblers.

* Add rocks, water, and coarse grit to tumbler.
* Let tumble for a week.
* Wash stones well.
* Repeat if desired for rounder stones.

* Add rocks, water, and abrasive grit to tumbler.
* Let tumble for a week.
* Wash stones well.
* Remove broken or sharp edged stones.
* Remove stones smaller than 1/4".

* Add rocks, water, and pre-polish to tumbler.
* Let tumble for a week.
* Wash stones well.
* Remove broken or sharp edged stones.
* Remove stones smaller than 1/4".

* Add rocks, water, and polish to tumbler.
* Let tumble for 10 days.
* Wash stones well.
* Remove broken or sharp edged stones.
* Remove any stones smaller than 1/4 inches.

* Add rocks, water, and ivory bar soap to tumbler.
* Let tumble for a few hours.
* Wash stones well.

I use about one tablespoon of grit or polish per pound of rocks in a rotary tumbler. I add water to just below the top layer or rocks in the coarser grits, and to just above the rocks in polish.

What do I need to get started?

  To get started I recommend the following:

  Some tumblers come complete with grit, polish and rocks.

What's the best type of rock to polish?

Agate and Jasper are very suitable for tumbling in a rock tumbler. Any rock with a hardness of 5-7 on the mohs hardness scale will generally take a nice polish in a rock tumbler. If the rough rock has a glassy lustre before polishing it will usually take a nice shine. If the rough rock has an earthy lustre before starting it will generally have an earthy luster after tumbling.

Do I need to use plastic pellets?

In most cases I recommend that plastic pellets not be used. Plastic pellets are sometimes added to a rock tumbler during the pre-polish and polish stages to cushion the rocks. Hard agates and jaspers turn out fine without using plastic pellets. Difficult to polish rocks like obsidian or glass generally will not take a high lustre polish in a rotary tumbler unless plastic pellets are used in the final 3 stages of tumbling. Rather than use plastic pellets, I keep some pea sized river stones on hand and add them to the tumbler if I want to fill up a bit of space.

What's The Best Rock to Polish?

It's hard to go wrong with  montana moss agate ! It rounds easily in the coarser grits, and it takes a high lustre shine in polish.

Polishing Petrified Wood

Petrified wood is generally composed of agate and/or jasper and/or opal and is polished the same as other agates and jaspers. Sometimes there are parts of a rock, or even whole pieces of petrified wood which will not take a polish. For example if a piece of petrified wood contains both agate and opal, only the agate will polish in a tumbler. If the opal shines up at all, it will be at best a satin texture.

Why aren't my rocks rounded after a week in coarse grit?

Jasper and Agate are very hard rocks, and so they take a long time to tumble.

I typically tumble rocks in coarse grit for a week. By then the grit has broken down and is no longer working, so I dump the slurry off the rocks, add more water and grit, and tumble some more. The grit might break down in 3-10 days depending on which tumbler I use, but a week is a good starting point.

Sometimes for jasper or agate we do this two to four times before we are satisfied with roundness of the stones. Then we do each of the other grits for a week to ten days each.

I typically add a tablespoon of grit to a pound of rocks. I add enough water to fill just to the bottom of the top layer of rocks.  I fill the tumbler more than 1/2 full of rocks, but less than 3/4.

Why didn't my rocks take a polish?
Tips on rock tumbling

 The number one problem people have with not getting a good polish on their rocks is impatience. If the earlier stages are rushed, then scratches are left in the rocks that the later stages cannot recover from... Typically I let coarse tumble for a week, medium tumble for a week, pre-polish tumble for ten days, and polish tumble for two weeks.

 The second most common problem is overfilling the tumbler. If the tumble barrel is overfilled the rocks tend take a lot longer to get rounded in the coarse grits. I prefer my barrels to be about 2/3rds full. Often times I will tumble two batches in coarse, and combine the batches for the rest of the stages.

Some rocks simply will not take a high polish in a rock tumbler. For example: Sandstone, calcite, onyx, rhyolite, and limestone.

Some types of rocks are brittle, which means that they tend to chip instead of polish. Glass, obsidian, common opal, and some banded agates fall into this category.

If the tumbler and stones are not cleaned well between stages, then the grit from a previous stage could be scratching the rocks and preventing them from polishing.

Plastic pellets get impregnated with grit so if they are reused in a batch with finer grit than they were originally used in they will scratch the stones. For example; if plastic pellets that were used in pre-polish are also used in polish the stones will not take a high gloss polish.

If you have a mixture of harder stones and softer stones in your tumbler, only the harder stones will take a high lustre polish. You can oftentimes get a nice polish on the softer stones by putting them back in the final polish after removing the harder stones. Best results are acheived when all the rocks in a batch have similar hardness.

If a stone breaks during the later stages of tumbling, the sharp edges will scratch the other stones, so remove it before the final stages of polishing.

I don't like polishing very tiny rocks in the final polish, unless I am polishing only tiny rocks.

I like to put a little bit of dish soap in the tumbler during the polish stage as it tends to make a nicer finish. Sometimes I even tumble the rocks with just Ivory and water after they are finished in polish.

I use more water during final polishing so that the rocks have more of a cushion and don't hit against each other so much.

Warm Regards,
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