Geodes are awesome. I love geodes. I sometimes spend literally hours online looking for the right ones or just watching the pretty geode photos.
Geodes are crystal-lined cavities inside a rock. They are often formed when a bubble of air is trapped inside hot lava, then it cools down and forms quartz crystals inside around the bubble. The crystals can grow inside the rock for a very long time, so the geodes can be a few milimeters or several meters wide (or even bigger).
This geode piece is agate. Agate is basically a form of chalcedony, which is a type of quartz (silicone dioxide), but unlike regular quartz it has characteristic banding. I wasn’t able to show the true beauty of this stone on the photos, but I hope you like it anyway. The tiny red beads are coral, added to cheer the piece up a bit.