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rocks for tumbling

How to Select Tumbling Rocks

Whether you buy them or find them, there are a number of qualities to look for in choosing rocks for tumbling. Hardness, types of fractures, grain, pitting, inclusions, and colorare all factors to consider when choosing what to put in your rock tumbler. Collecting tumbling rocks can be half the fun, with the added benefit that whatever you collect from Mother Nature is free. Getting outdoors is good exercise and can be even more fun if you go with others who are just as interested in finding interesting rocks. Collecting rocks during your travels makes for nice souvenirs once polished. If you don't have the time or interest to collect agate tumbling rocks or other stones on your own, buying tumbling rough saves a lot of time. No matter where your stones come from, though, consider the following when selecting and preparing rocks for tumbling:

Hardness. Mohs scale of hardness is the most generally accepted measure. The scale is named after its inventor, Friedrich Mohs, a German mineralogist who developed the scale 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. Not that it has anything to do with rocks, but a fun fact is that human skin has a hardness of about 1.5 and human fingernails are about 2.5. Who knew?!

Fracture. Stones are also classified by type of fracture: brittle, conchoidal, and along cleavage planes. An irregular, grainy fracture generally means that the material does not have sufficient structural integrity for good tumbling. Hard, amorphous materials like jasper or obsidian have conchoidal fractures in which the broken ends of a piece will have a glossy surface, curved like a seashell. Crystalline materials such as amazonite or "sunstone" feldspars will fracture cleanly along cleavage planes, which are lines of structural weakness in the rock. Stones of this type will often show further cleavage during tumbling and will have to be returned to the first grind unless great care is taken when breaking up the rough. Some materials such as rose quartz or amethyst, if roughly handled or exposed to wide temperature fluctuations, is subject to multiple fracturing. In this case, the material will have a multitude of hairline cracks instead of being glassy and clear. The stone may break or chip along one of these lines in tumbling, but more importantly, the stone, even if perfectly polished, will not be pretty. Discard any material with multiple fractures unless you want it for special effect and you're willing to risk the polish of the other stones in the batch. Softer stones like turquoise or malachite can also be given a fine polish by tumbling, but require special techniques. Even soft metals, such as silver or copper can be tumble polished with excellent results. Some jewelry makers tumble batches of silver with steel shot to obtain a beautiful satin finish with a fraction of the labor required for hand polishing.

Grain and Pitting. After hardness and fracture of the material have been tested, it should be checked for grain and pitting. This is especially important when you do not know exactly what type of stone you are working with. A coarse, sandy grain will not polish. Keep in mind that some of the shiny pebbles you find on a beach may have this kind of internal grain. Pitted stones, besides looking bad, will prevent other stones in your tumbling batch from polishing properly since grit from previous steps will be trapped in the pits and escape in the next step and might ruin the batch. If you cannot break up the material finely enough to avoid all pitting, don't use those rocks. Spot-check fractured materials for pits with a magnifying glass.

Color and Inclusions. Whether you buy tumbling rough or find it in the great outdoors, choose colorful materials. Since you are going to spend weeks tumbling and polishing it, choose a color you like and one which will be attractive when polished. Wetting a fractured surface will give you an idea of the finished color, and also will bring out the fine detail and invisible fractures which may not be noticeable when the rock is dry. These fractures may appear as dark lines after the rest of the surface dries. Some rocks may contain bands, dots, or inclusions (other minerals). That's OK as far as tumbling and polishing is concerned, as long as the dots, streaks and bands are approximately the same hardness as the rock's base material. For example, pyrite (fool's gold) inclusions in blue sodalite will polish very well since the two materials are similar hardnesses. But softer inclusions will wear away faster than the base stone and cause pitting. Large inclusions may separate from the rock altogether while tumbling. Some of the more interesting patterned stones are snowflake obsidian, Brazilian banded agate, lace agate, and malachite.

Cleanliness. Probably the most important characteristic of any tumbling materials is cleanliness. The best rule of thumb is GIGO— garbage in, garbage out. The tumbler is not a magic device for turning road gravel into precious stones. The final gem must be inherent in the original material, in its shape and hardness and coloration.  You will get the hang of this after trying a few batches.