Identification

• A rock tumbler is a machine that smooths and polishes small stones through friction. There are toy rock tumblers, more robust models that are used by hobbyists and industrial tumblers.
  
  

Types

• Toy rock tumblers are cheap and often come with starter kits of materials, including appropriate stones. The main problem with them is that their low power limits the kinds of stones that can be handled. These are always of a rotating cylinder design aligned horizontally.
  Another design that produces polished stones much more quickly is called a vibrational tumbler. The name is something of a misnomer, because the machines sometimes use ultrasound and other times use spinning/rotation around a vertical axis. They have the advantage not just of speed, but also of retaining much of the original, irregular shape of the stones instead of producing a collection of various ovoids.
  The main distinguishing features about tumblers are size and power. Tumblers are rated according to the load they can bear. This adds up to two things: Bigger tumblers can polish more rocks, and also a wider range of rocks.
  
  

How Do They Work?

• Unless ultrasound is involved, a rock tumbler is a cylinder lined with rubber or plastic. The cylinder is loaded with a batch of stones that are all of similar or the same hardness, a quantity of lubricant and some abrasive grit. The rocks slide past each other, and the grit provides the friction for polishing them. A typical and complete tumbling process will take at least 4 to 5 weeks and involves four steps, with each step involving removing the stones after several days, washing them of lubricant and grit, and then putting them back in with a new batch of lubricant and finer grit. The process in this way is similar to sanding something to a fine finish: You start with a coarse grade of sandpaper or grit to deal with the most severe imperfections, and then gradually reach the desired, fine grain. The coarseness of a grit is called mesh, with a lower mesh being more coarse.
  
  

Working Considerations

• A rock tumbler is a noisy machine, and as finishing a batch of stones will take roughly a month, the machines will prove to be a source of sustained noise. Where to do your tumbling is an important consideration you want to avoid a long string of complaints.
  
  

Advanced Techniques

• Some serious rock tumblers "perform" their stones. This means cutting the stones before putting them into the tumbler, to achieve more control over the results--producing spheres and tear drops, for example.
  
  

Geology

• While studying a tumbler guide about rock hardness is easy and direct, identifying the listed stones in nature for collection will require a little elementary geology. Being able to predict where those stones can be found will require yet more geological knowledge. This is important in rock tumbling, because collecting your own pebbles is a cheap source of raw material, and stones of a similar hardness must be used. For example, if you collect a random handful of small stones with the idea of polishing them for filling a decorative basket or jar, the granite bits will crush the limestone bits, ruining your work.