Frequently Asked Questions (FAQ)

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 abrasive grit to tumbler.
• Let tumble for one week.
• Wash stones thoroughly.
• Repeat if desired for rounder stones.

• Add rocks, water, and medium abrasive grit to tumbler.
• Let tumble for one week.
• Wash stones thoroughly.
• Remove any stones that are broken or have sharp edges.
• Remove any stones smaller than 1/4 inches.

• Add rocks, water, and pre-polish to tumbler.
• Let tumble for ten days.
• Wash stones thoroughly.
• Remove any stones that are broken or have sharp edges.
• Remove any stones smaller than 1/4 inches.

• Add rocks, water, and polish to tumbler.
• Let tumble for two weeks.
• Wash stones thoroughly.
• Remove any stones that are broken or have sharp edges.
• 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 thoroughly.

I use the following amounts of grit in a 3 Lb barrel:

Coarse:	4 Tablespoons (4 oz by weight)
Medium:	4 Tablespoons (4 oz by weight)
Pre-polish:	3 Tablespoons (2 oz by weight)
Polish:	2 Tablespoons (1 oz by weight)

To get started I recommend the following:

• A Rock Tumbler
• A Grit Kit
• Some tumbling rocks.

Some tumblers come complete with grit, polish and rocks.

Agate and Jasper are very suitable for tumbling in a rock tumbler. 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. 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.

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 or to fill up extra space. 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.

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 three stages of tumbling.

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.

Geodes polish the same as any other type of rock.

You may tumble them, or polish them on diamond grinding wheels, or polish by hand using sandpaper, or cut them with a saw and polish them on a flat lap. A rock tumbler is a good option.

You may tumble them whole, or broken. There might be soft spots and hard spots on your geodes so only part of the stone may polish. I think that adds to the charm of polished geodes. If your geodes are dual hardness, and you want to accentuate the difference, don't use coarse or medium grit, and start tumbling with just geodes for a few extra weeks. The soft rock will wear away leaving the areas of harder rock raised above the surface.

Obsidian is one of the most difficult stones to polish. Using the same techniques as for agate will produce a flat to semi-gloss finish on obsidian. During tumbling obsidian stones bump together and chip each other. Anything you can do to minimize the stones bumping against each other will help.

Plastic pellets are commonly used to cushion obsidian stones so they don't hit each other so much. Obsidian is one of the few stones that I recommend using plastic pellets with. I use plastic pellets starting after the coarse grind.

Covering the stones with lots of water will slow down the collision of the stones with each other.

I add a 3-4 tablespoons of dishwashing liquid to the tumbler during the pre-polish and polish stages. This lubricates the obsidian and also slows down the tumbling action.

Some people do a final polish on obsidian using Karo syrup instead of water. Some people finish obsidian using walnut shells in place of water.

A vibratory tumbler is the best option for finishing obsidian.

It's OK to put any kind of stone in the tumbler. The hardest rocks in the batch will take a high gloss finish. The very soft rocks will get tiny, and smooth, and very rounded, but they won't take a polish. And there might be some medium hardness rocks that come out semi-polished.

If it looks earthy before putting it in the tumbler it will generally still look earthy when you take it out. If it looks glassy before you put it in it will generally take a good polish.

A good rule of thumb is that if the rock will scratch a knife blade then it will take a nice polish.

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

What I typically do is to 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 three days or ten depending on which tumbler I use, but a week is a good starting point.

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

As far as amounts of materials go, 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.

The larger tumble barrel you have the quicker rocks get rounded.

• 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. If I have a batch that doesn't take a high luster polish I will generally redo only the pre-polish and polish stages.

• 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. There's no sense even trying. For example when I tumble wonderstone which is a soft rhyolite, I use only course grit or medium grit for one to three days which produces nice rounded stones which show the colors and banding well. Other examples include: Sandstone, calcite, onyx, and limestone.

• Some rocks have minerals of different harness in the same stone. Only the hardest minerals in the stone will polish. Examples include: Granite, unakite, mica schist, banded agates, and petrified woods.

• Some types of rocks are brittle, which means that they tend to chip instead of polish. Examples include: Glass, obsidian, common opal, and some banded agates.

• 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 achieved 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 have a feeling that they make the others less shiny. This is most noticable when the tumbler is less than half full. (I remove any rocks smaller than 1/4 inch in diameter in each stage after coarse.)

• I like to put a little bit of Ivory bar soap or liquid 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.

Benefits of Rotary Tumblers

• Easier to use
• Can start it and forget about it
• Less expensive to buy
• Gets rocks rounder

Benefits of Vibratory Tumblers

• Can put a nicer shine on rocks
• Polishes rocks quicker
• Does minimal shaping of rocks
• Handles fragile rocks better
• Uses less grit and polish

Disadvantages of Vibratory Tumblers

• Have to check it at least daily
• Hard to get the right slurry, especially in the coarser grits

I prefer to start out in my rotary tumblers to get the rocks rounded, and then transfer to a vibratory tumbler for the last stage of polishing.

Vibratory tumblers really outperform rotary tumblers in polishing difficult stones like obsidian, glass, or common opal.

A small 3# rock tumbler uses about 10-20 watts of electricity. So if you are paying $0.10 per KWH that would translate to about $1.00 worth of electricity per month.

I remember the day that someone suggested that I add sugar to my rock tumbler... It took about two days before the yeast had generated enough carbon dioxide to blow the lid off the tumbler. The mixture of sugar and polish hardened into a substance that very closely resembles JB-weld. I ended up replacing the benchtop because the goop couldn't be removed.

If you tumble fossilized material such as petrified wood, or whale bone, the stones will often release smells reminiscent of where they came from; perhaps a swamp, or a cedar log. Makes me feel for the people that tumble coprolite!