Geodes can be obtained by ascending, killing bats, finding treasure, completing the campfire quest, collecting premium daily chest, or buying them in the store.
There are 5 types of geodes from the most common to the rarest: Stone, Opal, Quartz, Magma, and Ender.
There's also 6th type, Special, which cannot be crafted and has same stats as Quartz geode.
Geodes can be merged to obtain better geodes.
Opening geodes rewards the player with powerups, schmepls, and experience.