Geodes can come in a variety of shapes, sizes, colors, due to variations in the mineral content. These geodes are all mostly some variety of quartz. The one in the back right is chalcedony, which is a microcrystalline form of quartz. In front of it you can see a nice geode with lovely agate banding around it. To the left of it is one that has pyrite in it - more on this one later. Back and to the left of it is another quartz geode, with little black crystals inside of it. And in the back is a large geode of beautiful quartz crystals.
Geodes are found in igneous or sedimentary rock. Cavities form within gas bubbles of lava; the material around the void hardens to form the outer layers of the geode. They can also form within sedimentary rocks, often limestone, that has been dissolved (usually be acids such as carbonic or sulfuric acid). Minerals are carried in by water into the cavities, resulting in the growth of crystals. When the rock surrounding the geode erodes, these geodes are left behind.
This is what geodes typically look like from the outside; often they are a gray or tan to brown color. Sometimes they even sort of look like a brain!
Geodes can have many different minerals. Most contain some variety of quartz, including clear quartz, rose quartz, amethyst, jasper, agate, chalcedony, calcite, celestite, pyrite and so forth. Sometimes you even get minerals deposited on top of other minerals. The only way to tell what is inside a geode is to break it apart. There is no easy way of telling what the inside of a geode holds until it is cut open or broken apart, however, geodes from any one locality usually have a more restricted variety of interior mineralization.
Image from inside Crystal Cave, the largest known geode in the world, located in Put-in-Bay, Ohio, USA.
Geodes are common in several locations in the United States, including Indiana, Kentucky, Missouri, Utah, and Iowa. Iowa even has the geode as the state rock, and has Geode State Park. The largest known geode was found in 1897 at Put-in-Bay located in Ohio. It is called Crystal Cave, and you can even go on a tour inside this 30 foot deep cave, with crystals as big as 3 feet (1m) wide.
Some creative person decided to fill this geode with pyrite! How can you tell it wasn't like this originally? Well, the pyrite is overhanging the edge of the cavity. Yet it was cut in half - if the pyrite had been there originally, there would be none sticking out beyond where the geode was cut! However its still pretty - just be aware geodes are sometimes modified.
Geodes are used for decorative purposes and jewelry. They are sometimes dyed, so if you see one for sale with very bright colors like neon pinks, blues, and purples, then it has been dyed. As seen in the above picture, other materials are stuck inside that don’t belong there! Sometimes objects are also placed inside, creating little landscapes. Geodes have been used both as slices and cut halves; care must be taken using the slices of geode as they can be somewhat fragile. But what wonderful and unusual jewelry they make!
Using geodes in jewelry work can be a bit tricky, but can have great results!
If there's a stone you would like to know more about - drop me a line! Have a mystery stone? Feel free to post a comment about it, including a link to a picture, and it could be featured on this blog!
Rocks, Minerals, & Fossils of the World by C. Pellant and R. Phillips. Published 1990 by Little, Brown and Co. ISBN: 0316697966
Wikipedia – Geode. http://en.wikipedia.org/wiki/Geode
Wikipedia – Crystal Cave (Ohio). http://en.wikipedia.org/wiki/Crystal_Cave_%28Ohio%29