An Everstone is a held item that can be given to a Pokémon to prevent it from evolving. When held by a breeding Pokémon, its Nature is guaranteed to be passed on to its offspring. It can be crafted, and can also be obtained as a tier 1 special drop.
Everstones can also be used to make Rock Gems.
This item is currently cannot be crafted, and is only available as a drop with certain versions of Pixelmon.