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The type of stones you use for rock tumbling is largely personal preference. But whether you collect them on your own or buy rough stones, for best results stick to stones of similar hardness but of various sizes when tumbling together. This will ensure they all take approximately the same amount of time to reach the proper smoothness, and that harder stones don't damage softer stones during the tumbling process. 

Many types of rock can be successfully polished into stunning gems, especially agates, quartz, and jasper. Agate is the most popular tumbling rough. It is generally inexpensive and can be tumbled with good results by beginners. Agate occurs in a wide range of colors which include: brown, white, red, gray, pink, black and yellow. The colors are caused by impurities and occur as alternating bands within the agate, providing interesting and pretty patterns.

Any rock with a hardness of 5-7 on the Mohs hardness scale will generally take a nice polish in a rock tumbler. Agates have a hardness of 7. Harder rocks usually result in a high-gloss finish. Softer rocks will get smooth and rounded, but they won't take a polish. If the rock looks earthy, it will generally still look earthy when you take it out of the tumbler. Mohs Hardness Scale is named after its inventor, Friedrich Mohs, a German mineralogist. The scale was developed in 1812. He selected 10 minerals of distinctly different hardness (hardness is the resistance of a material to being scratched) that ranged from a very soft mineral (talc) to a very hard mineral (diamond). Since you always want to tumble together stones of similar hardness for best results, it's important to understand this principle.

The key point to remember in collecting or purchasing rough material for your tumbler is: "quality in means quality out!" Don't waste time and money on grits trying to smooth out crevices and holes in a poor rock. Start out right by only using the type of rocks that are known to provide the best results when polished.

To start, fill the rock tumbler barrel between 2/3 and 3/4 full. Choose rocks of varying sizes, as this will promote thorough tumbling action. A batch of rocks that are all about the same size will often not tumble properly or grind very slowly. For a 3 pound barrel, a good range of sizes is from .25 inch to 1.5 inches. Lortone Rock Tumblers found here.

If you don't happen to live where agates can be found in stream beds or where quartz pebbles can be found along beaches, that's OK.  You can still enjoy tumbling rocks by purchasing the rough materials online. Although it's fun to collect your own rocks, it is often much more economical to purchase them. And remember that collecting on private property without permission is unlawful and that removing rocks from parks and most other types of public land is also illegal.

Rocks NOT to Tumble

It's just as important to know what type of rocks you should NOT tumble, as it is to know which ones with produce nice round and shiny gemstones. Sedimentary rocks such as sandstone, coal, limestone and shale are too soft or poorly cemented to polish into shiny gems. Metamorphic rocks that contain micas or have a "grainy" texture are also unsuitable - they will break up instead of becoming smooth. And, most igneous rocks do not tumble well because they contain several different minerals that wear down at very different rates.

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