How to Break Rock and Load the Rock Tumbler

If you plan to collect your own rocks out in the field, then learning how best to break rock and how to properly load the barrel of your rock tumbler are extremely important. Work gloves will protect your hands against sharp splinters of rock and broken edges, and safety goggles are a must. During the crushing or breaking operation, rock splinters may fly through the air. Goggles also will protect against metal chips that can fly off the hardened face of a hammer.

To break a rock, place it on a hard surface that is okay to damage (not on a sidewalk or concrete porch or a wooden deck - you will damage the surface). Wrap the rock in a piece of heavy cloth (such as canvas or denim) that completely encloses the rock and strike it with the hammer. The heavy cloth will help prevent shards from flying. Get a new piece of cloth when the one that you are using will not wrap the rock securely. Some people cut the top off of a small cardboard box and place the rock in the box before striking it with a hammer. The box will help contain the broken rock debris. Of course you can avoid the mess and hassle of doing this if you purchase prepared tumbling rough.

The sizes of the broken rock will depend upon the use of the finished pieces. Much jewelry is made from small pieces that would fit through a sorting mesh about 3/8 inch square. These small pieces are useful for rings, bracelets and items made from clusters of small stones. The large pieces are generally used for pendants, bola slides and similar items. When breaking pieces less than an inch across, it is wise to go to a smaller size hammer to avoid crushing or powdering the stone.

As you break up rock, look for pitted sections or sponge-like matrix material that must be discarded as unpolishable. Any rock with holes or pits that extend into the base material should not be used, since it will prevent clean stones from taking a nice polish. Chips and slivers are most prominent when breaking up conchoidally fracturing material— obsidian, goldstone or cullet (glass). Crystalline quartz materials such as amethyst or rose quartz also produce large number of slivers.

Don't try and tumble the slivers. All that happens is that they will become thinner, smaller slivers, which are unusable even if polished. They will only reduce the amount of material that could have been produced if all the broken pieces were chunks instead. Another reason to eliminate slivers is that they are the most likely pieces to break during tumbling and can ruin an otherwise perfect batch. All the shapes should be watched to some extent and controlled as much as possible while breaking the rock.

The tumbler is not going to dramatically change the original rocks' shape. For example, it will not remove a step (a break that occurs in materials that fracture along the cleavage planes, such as feldspar). You will find that some pieces of this material look like two cubes joined together. Since they are joined at about a 90 degree angle, the defect will not be removed by tumbling and the piece should be broken again to form two small blocks. Some stones will break up very neatly, but there will always be a certain amount of slivering and rock dust. It will seem that a lot of material is being thrown away before if even gets into the tumbler!

If the unbroken stones you collect are fairly uniform, with gross irregularities, they should be tumbled in a balanced mix instead of one consisting of all large stones, since the severe rounding and surface removal that occurs in an all-large batch will be unnecessary. A balanced load has a large number of small stones, fewer large ones, and the rest is a range of intermediate sizes. An even or uniform mix contains all small or all large stones.There is really no advantage to tumbling an entire batch of small stones. If small and large stones are tumbled together, the large stones will usually have more concave surfaces and irregularities. Although their polish will not be affected if all the stones are large, the results will be much more uniform and satisfying in shape, almost like irregularly shaped eggs. In a balanced load, smaller stones cushion the shock of collision between larger pieces, a role that must be filled by additives (such as plastic pellets) if all large stones are being tumbled.

The percentage of larger chunks — .75 to 1.5 inch or larger — in a balanced mix should be about 2 - 3 percent of the load by number, or up to 25% by weight. A balanced mix is generally recommended for a vibrating tumbler, although good results are achieved with an even mix of large, relatively hard agates.

After several weeks in a rotary rock tumbler or several days in a vibrating rock tumbler, the gemstones you end up with will be smaller in size and similar in shape to those you started with. The major difference is that they will be polished and the edges will be rounded.