Geodes are a kind of rock that are hollow on the inside, and minerals and crystals form inside them. They look plain on the inside but inside can be beautiful. Take a look at these images.
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.