How can rocks get hallow?

Yes

It is a small rock from the book "Tales of Beetle the Bard" which when rotated in the hand brings the dead back to life in the form of a ghost.

The density of rock B is greater because its volume is less.

No. A similar rock could be formed, but some characteristics would be altered.

Almost all rocks will undergo change at one point. So the answer is yes. metamorphic rock can undergo melting, becoming magma, when surfacing and cooling will become igneous rocks. Metamorphic rock...