Pokemon GO originally launched without evolution stones, letting Pokemon that required them in the main games to evolve just by using candy. However, the second generation of Pokemon introduced evolution stones to Pokemon GO, and they have become a focal point of the Celebi Special Research quest, giving players more ways to get them than ever before.
Previously, Pokemon GO players could only earn evolution stones by spinning PokeStops. Each PokeStop spin comes with a very slim chance of dropping an evolution stone, or players could be guaranteed one on the seventh day of a PokeStop spinning streak. With the addition of Celebi Special Research, Niantic has made it possible to get evolution stones by completing certain steps in the Celebi Special Research quest. Furthermore, it has also added evolution stones as possible rewards for completing Johto Festival Field Research.
In regards to Celebi Special Research, here are all the evolution stones players can earn, and which step they need to complete to get them:
As far as Johto Festival Field Research, players have a chance to earn all of the evolution stones by completing the Catch 3 Sunkern Field Research. Completing that Field Research is worthwhile for anyone looking to complete the Celebi Special Research anyway, as the quest has an objective to evolve a Sunkern into a Sunflora using the Sun Stone, so it never hurts to build up one’s stock of Sunkern candy.
Moving forward, it will be interesting to see if Niantic keeps evolution stones as Field Research rewards, or if they will be swapped out for more typical rewards. As it stands, some players may find that they have too many evolution stones taking up space in their bags, especially since Pokemon require the evolution stone and candy to evolve, so they may want to see them rotated out.
Regardless of how Niantic handles evolution stones in the future, right now they are available through Field Research and the Celebi Special Research quest, in addition to PokeStops.