The correct answer is: It illustrates the idea that good can come from a bad situation.
King embodies the abstract concept of Despair as a metaphorical mountain. This metaphore represents life and the tough moments one is ought to endure in order to survive. King means to deliver the existence of hope in life as a message, wrapping it in a metaphorical stone.
The explicit expression of that message would be: Stones can be digged from mountains, just as hope can emerge from a dire life.