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Can You Tumble A Geode?

Whether you can tumble a geode depends on its size relative to the size of your tumbler and also whether you have already cracked it open or not. Geodes are vaguely spherical or egg-shaped hollow rocks in which masses of mineral matter, including crystals, may be hidden. They look just like ordinary rock from the outside, but when you break them open, there are sometimes beautiful gemstones inside.

You can tumble a geode, but the results may not always be to your liking. If the geode is already in pieces, the inner crystals may be ground off, leaving only the greyish banded quartz rind. Even if the crystals don’t disappear, getting the tumbler’s sand and grit out of them would be impossible.

Many geodes are the size of a basketball or larger and may not fit into a hobbyist’s tumbler. There may not be much point in polishing a geode whole because the ‘rind’ may not take a polish well and may consist of rock that is not particularly attractive. The magic of geodes typically lies inside them, where perfectly smooth and shiny crystals are already formed.

Using A Tumbler To Polish Geodes

If you want to try tumbling a geode, clean it well first and ascertain whether it is made of soft or hard rock. Soft rock, like the limestone that coats many geodes, can disintegrate easily in a tumbler. Limestone is basically calcium carbonate and does not polish well. Some agate or chalcedony geodes with thick rinds may be tumbled with a measure of success. 

You should not tumble rocks with varying hardness together. Scrutinize the geode for any cracks and fault lines in its surface. If it has too many, or they look a bit deep, don’t put it in the tumbler. Over polishing a geode can lead to a loss of detail on its surface. Some geodes are packed with crystals, while others only have a crystal lining.

Many geodes have clear quartz crystals, while others have amethyst, celestite, or calcite crystals inside.  A lot of quartz geodes have concentric quartz layers of different types. Geodes may have agate, chalcedony, or jasper banding. There are many types of geodes, and it is helpful to know what kinds of rock they formed in when deciding to tumble them.

Tumbling is a long process that involves the use of various types of grits and polishing materials. It can take days or weeks to tumble rocks. Hobbyist tumblers have good quality motors that can last for years and usually have a metal frame. Non-granular rocks that are free of voids, cavities, and fractures are the most suitable for tumbling. This excludes geodes that, by definition, have voids and cavities.

Consult the manual for the tumbler to know how much water and what size grit to use. Your local mineral and gem society may have some helpful advice for polishing geodes, so be sure to check them out. They often have information or classes on lapidary, the art of cutting and polishing stone, and will also be able to tell you more about the composition of your geodes if you found them locally.

Can Geodes Be Damaged in A Tumbler?

Because they are essentially hollow rock shells, geodes tumbling with other rocks in a tumbler can crack open and may be damaged. Also, tumbling removes a significant portion of the rock. In the case of geodes, it may remove so much of it that there is hardly anything left. 

Some geodes do not have as much open space inside them as others. However, since you don’t know how big the void inside a geode is until you open it, you can’t always predict whether the geode will crack or break during the tumbling process. If a geode has cracks, the polishing materials could penetrate it and corrupt the contents.

A group of tumbled stones

It is not a good idea to tumble a geode you’ve already opened because the grit and sand used in a tumbler get in amongst the crystals. If this happens, the crystal bed can be ruined because it is so difficult to remove it. Some say that tumbling a hollow geode never ends well. Geodes from different areas have thinner or thicker rinds. You may get away with tumbling a small geode with a thick rind, but you don’t know if it is thick until you open it. If you open it, you shouldn’t tumble it.

Geodes don’t always contain crystals but can be lined with chalcedony that forms exciting patterns on the walls. An inner landscape is formed with a texture resembling grapes. The word for this type of formation is botryoidal. Sometimes the interior is lined with minute sparkling quartz crystals called druse. A few have even been found with liquid petroleum inside. One can only imagine what would happen should one of these break open inside a tumbler.

How To Clean A Geode

Geodes are often caked in dried mud, grit, or clay,  so you need to scrub them off to show them at their best. The easiest way to clean a geode is to simply wash it in plain water with laundry detergent or dish soap and then let it soak for two days in a tub of water mixed with a quarter of a cup of household bleach. Use denture cleaner and a soft-bristle toothbrush to scrub off any remaining dirt, and then rinse the geode in clean, warm water. 

Just be aware that scrubbing and chemicals can damage small crystal inclusions on the geode’s outer surface. 

Many geodes are quartz and can be cleaned with oxalic acid, also called wood bleach, if they have iron oxide staining. However, if you try using this acid on a calcite geode, it will form an insoluble precipitate that is hard to remove and may damage calcite crystals and carbonates. 

How To Polish A Geode

Once you have cut a geode open, you may want to polish it. You should not use a tumbler for this. Lapidary wheels can be used for polishing, but they are large and expensive pieces of equipment unsuitable for a hobbyist. You can use coarse moistened sandpaper to remove rough patches on the edges of an open geode. This can be a laborious process and requires a lot of patience, particularly if the geode is relatively large. 

Once it is smooth, you can use a heavy fabric like denim to get it shiny. You can buy commercial finishing polish for gems online or at certain hardware stores. Always wear safety glasses when polishing a geode to prevent eye injury.

A polishing machine called a vibrating lap or vibra-lap can be used to polish cut halves of geodes. It consists of a tray attached to a motor with a spinning weight that makes the tray move in a circular spinning motion. You can put various graded grits inside the tray to smooth the rock. Unfortunately, these machines can be pricey.

The cut edges of the geode halves must be placed on the surface of the tray. Leave the machine to run for around six to eight hours, checking periodically that the faces are being evenly ground.

You may be able to polish a geode in some instances with a rotary tool such as a Dremel. Clean the rock with soap and water and then secure it in a vice. Using the various sanding attachments for the Dremel, grind it smooth, and then use a polishing wheel and some polishing compound to make it shine.

Conclusions

Many geodes are not suitable for tumbling, and you run the risk of ruining them if you try. There are other better ways of polishing the rough edges of geodes that won’t damage them. The beauty of geodes is best appreciated by cutting them open to see what’s inside. If they are full of crystals and other delicate mineral structures, tumbling the cut halves is not recommended. 

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