Before the Island
Rose gave this ring to Bernard as a token of their commitment. Weeks later, while aboard Flight 815, Bernard's hands began to swell because of altitude, so he gave his ring to Rose for safekeeping. Rose wore the ring on a chain around her neck. When Bernard went to the bathroom in the back of the plane, the plane encountered turbulence and crashed. Bernard was trapped in the tail section of the plane. ("Pilot, Part 1") ("Walkabout")
On the Island
Rose holds Bernard's ring in ("Walkabout").
In the initial chaos following the crash, Rose was resuscitated by Jack. She then began a vigil of sitting on the beach looking out into the ocean, clutching the ring and rocking back and forth. She revealed to Jack that she believed that Bernard was still alive, and was praying for his return. Jack did not believe that her husband could be alive, but Rose's hope was unwavering. ("Walkabout")
Forty-eight days later, Bernard returned from the other side of the Island, where he had been surviving with the tail section survivors, and reunited with Rose. The two embraced warmly. Rose eventually returned Bernard's ring to him. ("Collision")