At the end of her letter, Gemma lets Ty in on a recurring dream she's been having about her time in Australia. In the dream, she's on the path inside the Separates, digging a hole with her hands. When she's made it big enough, she takes off the ring he made for her and puts it in the ground. "I'll start to sprinkle the earth back over it, and I will bury it," she says. "Back where it belongs" (113.2).
Dreams are pretty important throughout Stolen. Gemma dreams of what her parents are doing at home to deal with her absence, while Ty has violent nightmares about being taken away from his dad's farm to the city. In these dreams, the focus is on the past, but the one that ends the book seems to focus on the future.
On one hand, Gemma burying the ring is an act of symbolically returning Ty to the land where he "belongs." He'll obviously be going to jail in the near future, a place where he clearly won't feel he belongs. But in her mind—via her dreams—she can keep the part of him that's passionate about the land and nature in the earth where it should be.
There's another interpretation, though. Perhaps Gemma burying the ring symbolizes not just returning Ty to the land but the rebirth she hopes he'll experience as a result of this whole ordeal. "One day they'll let you out of that dry, empty cell," she says. "You'll return to the Separates, and you'll feel the rain once more. And you'll grow straight this time, toward the sunlight. I know you will" (112.11). Gemma hopes that someday the part of him that is compassionate and beautiful can thrive and grow, if only he'll let it.