Geodes are formed when groundwater deposits minerals within cavities in rock. Geodes form very slowly, filling the cavities with quartz crystals from the edges in. The quartz can range from large quartz crystals to microcrystalline quartz, often called chalcedony. Quartz agate can also occur in geodes. Colors of deposits vary according to the minerals present in the groundwater. A nodule is a geode that has filled completely with deposited minerals, leaving no hollow in the middle.
Obtain raw geodes via rockhounding or by purchase. Geodes can be dug for non-commercial use from public lands such as Rockhound State Park in New Mexico, the Hauser Geode Beds, the North Black Hills Geode Beds and the Cinnamon Geode Beds in California, and in various areas of Iowa. You can purchase geodes from rock and mineral shops and from online sources.
Place the unopened geode in a sock or in a nest of cushioning materials. Use a rock hammer to sharply strike one area of the geode. Repeat until the geode splits open. Alternatively, you can open the geode by using a chisel and hammer or a rock saw. To use a chisel, place the cutting end of the chisel on the exterior of the geode; strike the hammer to the other end. Whichever method you choose to open the geode, wear gloves and safety goggles and keep others out of range of possible flying shards when the geode breaks open.
Examine the interior of the geode when it is broken open. If the rock has a cavity toward its interior, it is in fact a geode. If it proves to be solidly filled with mineral deposits, it is a nodule. There is no way to tell if a rock is a geode or a nodule without breaking it open.
Using a rock saw to open the geode takes longer and may cut through some of the crystals. However, cutting is a good way to reveal possible banding in agate deposits.