What Size Rocks Are Best Used In A Rock Tumbler?

Summary: Use a combination of small rocks, medium-sized rocks, and large rocks. Don't use any larger than half the barrel diameter.

The Two Commandments For Rock Sizes While Tumbling

There are two basic guidelines for loading stones into your rock tumblers:

1) Put a wide variety mixture of rocks into the barrel.

2) Don't go too big - keep your largest rock size well under a quarter the radius of the barrel opening.

Rule #1 IS MANDATORY!   It's not optional.  Keep the second rule in mind if you are a rockhound into tumbling bigger rocks - most people don't fit this category.

Choose rocks of all sizes to tumble

For small tumbler bottles with a 3-lb capacity barrel such as the ones pictured above, look for rocks as small as 0.25" - you can go slightly smaller - and don't go above 1.5" rocks.  These are guidelines - experiment to see what works for you.

For the larger barreled rock tumbler, you still need a good mix of rock sizes to get the results you want.  Just like with the 3 lb tumbler, you want rocks as small as your pinky in the mix.  However, you can move up in size as well with the larger-diameter rock tumblers.  You'll find it pretty simple to tumble two to three-inch rocks in one of these rock tumblers.

It takes a long time to produce good quality tumbled stones. With a little patience, good initial rock selection, and some luck, you'll turn out beautifully-tumbled round rocks you can be proud of.  Have fun!

You need small rock pieces in the mix for the best results while rock tumbling.

As a refresher - we call the action when two pieces of rocks rub into each other "grinding".

Added grit particles (such as ceramic media) catch between these tumbling rocks, which causes them to grind against each other. Increasing the number of contacts increases the rate at which they grind.

You should load up your tumbler barrel by adding different-sized rocks to it. The variety of sizes is key. Make sure you have a variety of rock shapes and types available so you can create a good mixture to maximize the chances of grinding per tumble.

Ceramic media: These cylindrical ceramic pieces are about the size of a bb from an air gun. They're good for tumbling any type of rock but especially useful for balancing out large particles in a tumbler. Remember - we are trying to maximize surface contact between rocks so these ceramic pieces can fill that need if you have only a few rough small-sized rocks.

Ceramic rollers are the most commonly used shapes for grinding stones. They're cylindrical, which helps them move around inside the tumbler and deliver abrasive material to the surface of the stone. They can be rinsed after each use and go through every step of the polishing process.

Keep ceramic cylinders on hand for all rock tumbling projects.  Throwing in 5% or so (by weight) will really enhance your results from rock tumbling.  And it works well with both vibratory and rotary tumblers.

Here's a good article for more information about getting started and utilizing ceramic media.

Producing Tumbles With Big Rocks

Some people like to make very large tumbled stones - because smaller stones just don't do it for them.  This is an exercise in patience and time - it takes a LONG time to rock tumble these large stones.

You can make them by running large stones through the coarse grinder multiple times. They can take up to several months to get nice and round.

You can easily tumble small stones if you're patient enough. However, don't throw too many of them into the tumbler at once because they may jam in the barrel, smash your small material and make it worthless or even cause cracks in the large rocks. This is definitely not a numbers game - take it slow. No one wants to damage their semi-precious stones.

Select your stones carefully if you intend to create rock tumbles with big rocks. You don't want to waste 5 or 6 weeks only to get a broken product as a result.  Select the right rocks - those with large indentations will exponentially increase the time it takes to smooth your giant rocks. Likewise check for hidden cracks - these will become real cracks after a few weeks in the tumbler.