The art of rock polishing has been around since time immemorial. It was there before man or animal walked the planet. It was done by nature. The rains were one of the greatest rock polishers. The constant erosion that occurred on coarse rocky surfaces gave way to some amazing masterpieces. But the winds of commerce have engulfed this phenomenon. Today it has become a profession. Rock polishing is a machine intensive industry. So much so that, there are institutes that teach courses to students interested in the same.
Rock polishing is achieved via a process called 'tumbling'. A tumbler consists of an electric motor, a barrel, and a frame with rollers. A pulley and a belt connect the rollers to the motor. To begin with, the barrel is filled up to almost three quarters with stones of about 1 inch in diameter. Silicon carbide is then added in a proportion that is half the total load in the barrel. Water is also added. The basic procedure is to tumble the rocks with progressively finer grits and polishes until the desired shape and shine is achieved. As the barrel rotates, its contents collide, wearing down the stones. The stones then begin to erode. The rate at which the stone erodes will depend on its characteristics. Every day, stop the machine, remove some stones, and wash them under clean water. Then allow them to dry and inspect them with a small magnifying glass under bright light. Clean out the sludge in the barrel. If the stones are pitted or cracked, return them to the tumbler, add the right amount of silicon carbide and water, and repeat the process. When your samples do not reveal these pits, cracks, and hairlines, remove all the stones. It may take 4 to 6 weeks to finish a batch. Once the stones are perfected, comes the final part. A tablespoon of cerium oxide per pound of stones is to be added. Tumble for another week.
There are different types of tumblers in the market. A Rotary Tumbler is easy to use, less expensive, and gives the rocks a good basic shape. A Vibratory polishes rocks quicker but is cumbersome to use. Cost wise they range from about $50.00 to $800 depending on the usage. A stone-tumbling machine can be found in most hobby or lapidary supply shops.
There are some stones that are very difficult to polish. Only course grit or medium grit is used for one to three days when Wonderstone, which is a soft rhyolite, is tumbled. This produces nice, rounded stones which show the colors and banding well. Sandstone, calcite, onyx, limestone, glass, and obsidian also fall into the category where there may be veins in the rock that will not polish. Some types of rocks are brittle, which means that they tend to chip instead of getting polished. Other reasons for the stones to remain unpolished are process-related. If the earlier stages are rushed, then scratches are left in the rocks that the later stages cannot recover from. If the tumbler and stones are not cleaned well between stages, then the grit from a previous stage could be scratching the rocks and preventing them from polishing. If you have a mixture of harder stones and softer stones in your tumbler, only the harder stones will take a high luster polish. Best results are achieved when all the rocks in a batch have similar hardness. If a stone breaks during the later stages of tumbling, the sharp edges will scratch the other stones, so get rid of them before the final stages of polishing.
Agate and quartz are very suitable for tumbling in a rock tumbler. Any rock with a hardness of 5-7 on the hardness scale will generally take a nice polish in a rock tumbler. If the rough rock has a glassy luster before polishing, it will usually take a nice shine. If the rough rock has an earthy luster before starting, it will generally have an earthy luster after tumbling. Tumbled and polished rocks have been very popular with hobbyists and collectors for generations. Finished stones can be collected and traded, or used for beautiful ornamental pieces.