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Tumbling Stones

Using a Tumbler

Using a tumbler is the easiest way to polish unusually shaped rocks. However, it is still a lot of work. Polished, tumbled stones are referred to as baroque stones, because of their nonstandard shapes. Special hardware is available from lapidary supply houses to mount these lovely stones in jewelry for your family and friends.

A tumbler consists of a barrel with a motor to turn it. A simple tumbler can be made from a jar or metal can with a bolt inserted through the center of the lid. A barbecue rotisserie motor can be used to turn it. Tumblers are noisy and can be messy, so they're best kept in the garage. Placing it on a piece of rubber or Styrofoam or putting a box over it can minimize the noise and mess.

There are two stages to the tumbling process: the grinding stage and the final polishing stage. Grinding removes the rough edges and cavities from your stones using powdered abrasives. The coarsest abrasives have the smallest numbers, like 60 grit, and the numbers get larger as the abrasive gets finer, up to about 1,000 grit. The polishing stage uses a polish such as alumina or cerium oxide to make your gemstones shine.

Cleanliness is most important when using a tumbler! After each stage of the tumbling, the tumbler and all the stones you are polishing must be carefully cleaned with soap, water and a brush. It is a good idea to have a separate barrel for each type of abrasive and polish, so you don't have to clean them between steps.

Each tumbler comes with a set of specific instructions, so I'll just describe the basic process. Begin the tumbling process by selecting stones that similar in size and hardness. Quartz gems like jasper, agate and amethyst usually turn out very well, but they take a long time.

Fill the tumbler barrel a little more than halfway with stones and the coarsest abrasive, then add water to almost cover the top layer of stones. Operate the tumbler for a week or so, checking periodically to make sure everything is working properly and the water ins't leaking out.

When the coarse abrasive has worn the sharp edges off the stones and there are no cavities or rough places to be found, clean the stones and the tumbler barrel with soap, water, and an old toothbrush. Replace the stones in the tumbler with 200 grit abrasive and water. Allow the tumbler to work until the scratches on all the stones appear uniform.

Clean the stones and replace in the barrel with 600 grit abrasive. if you have the patience and want a very lustrous batch of gems, run them one more time with 1,000 grit abrasive. After carefully cleaning and drying the stones, you should observe a uniform sheen with no large scratches.

For polishing. the tumbler is charged with alumina, tin oxide, or cerium oxide. You might include a carrier, such as grains of walnut shell or small plastic beads that rub against the polished stones and carry the polishing medium into every small crack and depression. Some folks also put a teaspoon of Tide detergent and same sugar at this stage. The polishing process is completed when the stones look the same wet or dry.

Choosing Rocks for your Tumbler

We have discussed the methods and materials needed to polish rocks in a tumbler. Now let's talk about what rocks we would like to polish and where to find them! As you select stones for your tumbler, keep in mind these properties: hardness, cleavage, size, shape and beauty.

Common quartz gems such as agates and jaspers often show wonderful patterns and colors when polished. You may find these types of stones are in dry or flowing rivers and creeks. The stones will probably have been worn and shaped by running water and sand, and may be more than halfway polished when you find them. Carry a spray bottle to wet the stones so they show their lovely colors, patterns and textures.

Tiger-s-eye and chrysoprase are special quartz varieties found in Africa and Australia that are lovely when tumble polished. Tiger's-eye seems to change color from gold to brown when rotated in bright sunlight, and chrysoprase has a lovely green color caused by the element nickel.

The crystallized varieties of quartz are also good for tumbling. Amethyst and citrine often occur as intergrown masses of crystals that can be broken into small pieces for tumbling and make lovely jewelry stones. The small crystals that form in veins are perfect for earrings and pendants. Larger single quartz crystals can also be tumbled, but are usually considered more beautiful when left in their natural state.

Some banded calcite is known as onyx, a term more properly applied to a banded variety of quartz. Calcite onyx can be tumbled or carved, and is often carved into figurines in Mexico. The material is quite soft (Mohs 3), so it will not take very long to grind it to dust in the tumbler. Check it frequently! Sand or dirt left in the tumbler will scratch this material and prevent it from being polished.

Stones tumbled together should be of a similar size, and must be of the same hardness. Don't try to mix agates with apache tears, for example.

Stones with a definite cleavage such as feldspar, flourite and calcite will break into pieces when tumbled unless you take special precautions to cushion them using pellets or sawdust, and even then you will have problems.

The most common use for tumbled stones is to glue them into findings to make jewelry. Findings are small metal settings available from your local rock or lapidary shop or by mail. Choose small stones for use in jewelry, so the finished piece will not be too heavy to wear comfortably. Also give some thought to the shape of the stone. Teardrop-shaped stones are easiest to sett, but any stone you use should have a pleasing and interesting shape that will lend itself to your jewelry concept.

TUMBLING COMPOUND QUANTITIES
BARREL CAPACITY STEP 1 STEP 2 STEP 3 STEP 4 STEP 5 INTERIM WASHES
Size Max Volume Coarse Grind Medium Grind Pre-polish Polish Burnish (Soap) (Soap)
pounds cups TABLESPOONS
1.5 1 3/4 4 4 5 5 1/2 1/2
3.0 3 4 4 6 6 1 1
4.5 5 8 8 8 8 1 1/2 1 1/2
6.0 7 10 10 12 12 3 3
12.0 15 20 20 24 24 4 4