You'll want the Fabric API.
Adds stuff to worldgen that I thought might be interesting.
Cave Paintings:
These spawn underground. They don't serve any purpose other than to be mildly interesting. There is a small chance a cave painting may generate as a cave rune, and when broken, will drop runic dyes. When combined in an anvil with an enchanted item, these dyes will change the color of the enchantment sheen on your item.
Both cave paintings and cave runes may be silk-touched if you'd like to relocate them.
Imcompatibility
In order to work, this mod uses mixins clientside, namely in ItemRenderer and ArmorFeatureRenderer. It's pretty likely to be incompatible with other mods that change those classes as well.