Symbolism, Imagery, Allegory
Jude, the queen of freaking out about obscure diseases, has a question for her dad. "Isn't there a disease where the flesh calcifies and the poor afflicted person is trapped within their own body like a stone prison?" (4.64) The answer is yes, he says. "It's called FOP and it's extremely rare, Jude. Extremely, extremely rare." (4.66)
He's missing her point, though. For once, Jude wasn't being a hypochondriac. Her comment wasn't literally about having FOP. You see, it's a metaphor for her and Noah and her dad, who are all trapped—"our real selves buried so deep in these imposter ones." (4.67)
By the time this conversation takes place, Jude has already decided (for entirely different reasons) to start sculpting in stone. At first, she thinks she wants to make a sculpture of her mom, but when she begins, the piece that emerges is one of her and Noah.
Working on it brings up a lot of emotions that she's been trying to ignore, especially her guilt at having messed up his art career. "I take the hammer and crush it into the chisel. I have to get him out of here. I have to get both of us out of this f***ing rock," she says. (6.73-6.75) "I'm pounding on the stone now, cracking into it, opening it all up." (6.79)
That's right, girl. Let it all out.
When she's finished, the sculpture is broken in two. "NoahandJude becomes Noah and Jude," she observes. (6.109) She has released herself and her brother from each other—and from their stone prisons—so they're both free to move on with their lives.
Oh, one last thing: Jude's move from sculpting in sand (which is washed away by the waves) to stone (which is, obviously, a lot sturdier) suggests that she's grown way more confident in her work.