Geodes are a kind of rock that are hollow on the inside, and minerals and crystals form inside them. They look plain on the outside but inside can be beautiful. Take a look at these images.
Geodes can form in any hollow area such as tree roots, animal homes, or volcanic rock that is hollow because it was full of gases.
Maybe there is a tiny hole in the rock surface and a drip is getting in there, allowing deposits of minerals to get inside and to grow over time.
When a geode is cut open, it reveals the minerals inside and the patterns they have grown.
You can’t tell what’s inside a geode until it is cut open. Some geodes for sale have been man-made, or altered somehow to get all the colors and formations.