https://www.polygon.com/2018/11/16/18097471/pokemon-go-update-sinnoh-stones-fourth-gen-evolutions

A Pokémon Go update has added a new evolutionary item into the game: Sinnoh Stones, items that allow Pokémon from previous generations to become their Sinnoh-region forms. But for collectors looking to evolve their first-, second-, or third-gen Pokémon into special, exclusive fourth-gen monsters, it may take some time in order to collect enough Sinnoh Stones to do so.

Only one Sinnoh Stone is required to evolve compatible Pokémon, alongside 100 candies. Thus far, the only way to obtain these items is through completing field research. After collecting seven consecutive days’ worth of stamps, received by completing daily tasks, some players have found their research breakthrough rewards to be the elusive new item.