The bone's particular shape and the presence of two round eminences (corresponding to the so-called sesamoid bones) indicate that it's DIO's first metatarsal bone, i.e. one of the bones within the foot behind the big toe.
Before the events of Stone Ocean, DIO asked Pucci if he will betray him someday with the notion of how he could take The World any time he wanted with Whitesnake. Pucci explained that he loves DIO as he loves God, and how he had never thought of such a thing. DIO apologized for insulting him and handed him a bone that he ripped out of his own body as a token.
During the events of Stone Ocean, after Pucci enlists Sports Maxx's power to bring it back to life, the bone becomes infectious. Anyone who touched it would mutate, slowly germinating until becoming a sort of plant.
The Green Baby then sought out Pucci, and, upon finding him, fused with his Stand in order to create C-MOON.