DIO's Bone
The term DIO's Bone (DIOの骨 DIO no Hone) refers to a bone of the leg belonging to DIO, entrusted to Enrico Pucci before the events of Part VI: Stone Ocean.
Summary
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.
Once plants, the nutrients they absorb from sunlight were given to a baby-like creature referred to as DIO's homunculus, or the Green Baby. Upon the birth of the Green Baby, a part of DIO's soul was reborn into it.
This part of DIO then sought out Pucci, and upon finding him fused with his Stand in order to create C-Moon; as a fusion of DIO's Stand The World and Pucci's Whitesnake.
This Stand seems largely to have been a placeholder for a final step, as DIO slowly assumed Pucci's mind and they became one. C-Moon's gravity manipulation allowed Pucci to perform the final step in the evolution, giving him Made in Heaven: a Stand able to accelerate time.
In this final form Pucci demonstrates several of DIO's tactics, such as using his control over time to launch an attack of speeding knives at the heroes.
Notes
It should be noted that even though it is considered DIO's bone, it is actually a bone from the body of Jonathan Joestar, whose body Dio stole, replacing Jonathan's with his head, at the end of Part I: Phantom Blood.
According to the rules established in Part III, it is because of this that the Green Baby, and later on Enrico Pucci and his twin brother Weather Report each receive the Joestar Birthmark on the back of their necks.
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.
