Rift Quest:Pristine Samples  

In 'Dayblind Hollows, kill Deadwater cultists to find 6 Frost Etched Keys to open Deadwater Chests to get 6 Pristine Tidestones. The chests are in the caves and respawn quickly, so you could get all 6 updates in one cave, but the chokepoint is the Keys, which are an uncommon drop.