The Memory Ring is a piece of tech that was created by Cisco Ramon in order to recreate J'onn J'onzz power of memory restoration.[1]
History
At some point Cisco Ramon created the ring and gave it to Laurel. Laurel used the ring to help Mia Queen regain her pre-crisis memories.
Later a hooded figure used their own memory ring to restore JJ Diggle's memoreies of being Deathstroke.