Rotary Rock Tumbling: Choosing Material

Part 2 of 6 – making the right material choices

If you are just beginning and do not already have several successful confidence building tumbles under your belt, or if you’ve gotten off to a rough start and need to start over, we strongly recommend using a hard stone such as agate or Jasper that doesn’t have a lot of odd shapes with recesses or pitting. Smooth(ish) and round(ish), solid and hard and free from pits is a great place to start.

What follows next in no particular order is a bullet list of tips, tricks, and advice on choosing material for your rotary rock tumbler, contributed by the Rock Tumbling Hobby Forum community.

  • Don’t overlook the beach! If you have access to a beach with round smooth pebbles of varying sizes this can be a fun and inexpensive way to gather material for a tumble load, and even get the family involved! Most beach pebbles are of a hardness that has withstood the natural shaping of the ocean together, and tumbling will certainly bring about a visible transformation. If it’s a mirror shine you seek this might not be the right approach but it will certainly be one of the most fun.
  • If you don’t have a local source of smooth beach pebbles, and can’t find quality agate or jasper to collect in your area, search the online shops and be prepared to pay a little more for good quality sorted rocks from a reputable supplier. If you need ideas on where to find good materials, and which vendors can you trust, ask the Rock Tumbling Hobby community for help.
  • The internet is full of advertisements for very colorful and fancy stones for tumbling such as amethyst and rose quartz, labradorite and calcite etc., but many of these are soft stones requiring skill and experience. Our best advice is don’t add a high degree of difficulty upon yourself and try to avoid these flashy and expensive soft materials until you have a few successes under your belt. Many beginners get discouraged and give up when trying to tumble these fancy soft stones often shipped with new rock tumbler purchases.
  • One of the worst and most common mistakes beginners make is to mix hardness of material. Familiarize yourself with the Mohs hardness scale as it relates to the hardness of rocks we encounter in the lapidary hobbies, and this will help you greatly when it comes to choosing materials to tumble together.
  • Take the time to learn about the rough you’re about to try to tumble before you buy or collect it, and if you’re still not sure about a material, ask for advice on the RTH Forum.  Even if you have painstakingly chosen a tumbling load of rocks based on matching material hardnesses, some materials such as Leopardskin Jasper (rhyolite) will simply not achieve a high shine no matter how hard you try. This doesn’t represent a failure to tumble the material properly, more of a lack of understanding before you began.
  • Mohs hardness is important but there is more to be learned. Obsidian and feldspar materials like labradorite and unakite share a Mohs hardness in the 5 to 6 range but their mineral formations and breakage is very different and they are very incompatible to tumble together. Again, until you get some successes under your belt, it’s best to sort your materials by hardness and by like-kind.
  • Where did my favorite rock go? If you put a significantly softer stone in with extremely hard others, it is likely to completely disappear very quickly.
  • The amount of time a rough grind takes is determined by the worst rock that you put in the barrel. In other words, if most of the rocks you put in the barrel are already smooth with no major cracks or recesses, the handful of clearly flawed rocks you put in the same load are the ones that will add the most tumbling time on trying to get them fixed and ready to move to the next stage.
  • If you have lapidary rock grinding equipment seriously consider pre-grinding some or all of the stones that you have decided to tumble. Focus on getting rid of major cracks or recesses that may or may not be able to be removed by the rough grinding cycle. Doing this can make an enormous difference on the amount of time spent in rough grind.
  • The old computer programming adage garbage in garbage out is very appropriate to remember when setting up a rock tumbling load. It’s very exciting to tumble rocks that we have collected ourselves but the quickest and smoothest path to success almost always comes from purchasing presorted tumbling rough from a trusted source.
  • Soft stones can still have bits of hard matrix attached. One trick you can try is to run them at 500 grit AO for a few days and then separate the ‘problem’ pieces before they ruin your batch.

Now that you have a more basic understanding of rock materials and have made your selection or purchase, it’s time to move on to Part 3 of this series: Rotary Rock Tumbling – Loading the Tumbler Barrel

If you would like to offer feedback about your own experiences, pro-tips, tricks and advice, please post in this thread in the Rock Tumbling section of the RTH Forum, and they might eventually make it into the appropriate category in this article series.