The Cross Necklace was worn by Sarah Miller and later given to John Rambo to thank him for his kind efforts. Rambo wore it as a bracelet and it helped give him a new lease on life. It was a small cloth necklace with tiny wooden crosses on it.
Sarah Miller wore the necklace for the first part of the film, and when they arrived at the dock after Rambo gave them a free boat ride to Burma, Sarah gave Rambo her cross necklace as a token of her appreciation of his kindness to them. John wrapped it around his wrist and wore it as a bracelet for the rest of the film. Possibly as symolism for newfound faith in a higher power or himself.
There are two special necklaces worn by Rambo in the films. The first and most popular one is Co Bao's Buddha Amulet made of apple green Jade. The cross necklace was chosen to continue the religious symbolism from the first necklace, which Rambo gave to a boy in Afghanistan in 1988.