Choosing what not to tumble together

One of the most common beginner mistakes is throwing a mixed bag of random stones into one batch and expecting even results. Stones of very different hardness should not be tumbled together, softer material gets pitted both by loose grit clinging to the harder stones and by the harder stones physically banging into it. Try to batch material by hardness and general toughness as closely as you can. Very soft material, generally anything below about 5 on the Mohs scale, or material with visible internal fractures, often tumbles poorly no matter how careful you are, and is frequently a better candidate for hand polishing on a flat lap where you control the pressure directly rather than leaving it to bounce around a barrel for days.