Getting Started with Rock Tumbling

If you've been to a gift shop in Oregon, you have likely seen the tumbled rocks. They come in small bags and large buckets, and most importantly, are very easy to make. Tumbling rocks, is probably the most common way people get involved in the exciting hobby of rockhounding. All it takes is a rock tumbler, a few spoonfuls of grit and a bit of patience.

First things first – to tumble rocks you need a rock tumbler. On a very superficial level a rock tumbler is a bucket that is either shaken or rotated over a period time. There are, in fact, several plans for making rock tumblers from plastic buckets on the internet. Nevertheless, most of us start with a commercially made tumbler. There are two major varieties on the market: rotary tumblers and vibratory tumblers.

Rotary tumblers consist of one or more sealed drum resting on two rotating shafts. As the barrel slowly turns, the rocks gently bump against each other. Vibratory tumblers, on the other hand, generally come with one upright drum supported on several springs and connected to a vibrating motor. As the drum shakes form side-to-side, the rocks bump into each other. Both types have their advantages and disadvantages:

Lortone Rotary Tumbler

Vibratory Tumblers

Rotary Tumblers

Advantages

  • Tumble rocks faster
  • Use less grit
  • Doesn’t crush delicate rocks

Advantages

  • Easier to use
  • Run quieter
  • Less expensive
  • Product more rounded shapes

Disadvantages

  • More expensive
  • Noisy
  • A bit more difficult to use

Disadvantages

  • Take more time per cycle
  • Use more grit and polish
  • Can leak
  • Need to be filled to a certain level to work

Technical note: The process happening inside the tumbler is called “lapping”. As the rocks roll and bump into each other the little pieces or hard grit roll between the surfaces, producing microscopic fractures. As the size of the grit goes down, the fractures become progressively smaller, thus producing a highly polished surface. This is, in fact, the same process used by the lens makers to produce extremely accurate lenses and telescope mirrors (under much more controlled conditions of course)

In addition to the rocks, to get started you would need several gauges of grit and polish. Grit consists of small sand-like particles of silicon carbide – a very hard material that can “cut” into most types of rock encountered in the hobby. Most hobbyists use up to three different sizes of grit. The so-called 60/90 grit is used for the rough shaping. After this step is completed, 120/220 grit is used to remove the scratches. The next step is pre-polish using the fine 500 grit. Next a very fine aluminum or cerrium oxide powder is used for the final polishing step. Some hobbyists finish the process by running the rocks through a burnishing step with clean water and some non-abrasive soap to wash out the grit from the cracks and give the rocks some sparkle.

The amount of grit used depends on the size of the drum and the type of the tumbler. It's best to refer to your tumbler's manual, but a good starting place for a rotary tumbler is to use 2 tablespoons of grit or polish per 1.5 pounds barrel size. For example, a 3 lb tumbler would use 4 tablespoons of grit, while a 12 pound drum would require 16 tablespoons.

Rock tumbling is an exciting way to get involved in the hobby. You can purchase a good-quality tumbler and enough grit to run it for a year for under $100 from a reputable lapidary supplies dealer or your local hobby store, and the rocks can literally be collected in your backyard. Alternatively, if you are lucky enough to have a lapidary club in your area, it's might be well worth it to join it, or at least attend some meetings. Club members are more than happy to help newcomers with a good advice and some great tumbling material. You never know, you might even score a used tumbler.