Rift Keystone Fragment

Rift Keystone Fragments, usually called 'Rift Keys," are materials required to open Nephalem Rifts. Five of these are consumed each time a Rift is opened, and new ones can only be obtained by completing bounties. There are usaully 2 RKFs inside each Horadric Cache,

Rift Keystone Fragment tooltip, seen in the Bount reward window.