Kain is probably the only beast-tamer alive that can so easily identify the identity of the orb they chipped a piece off of. However, there was one crucial feature that made its identity clear to him.
On the seamless purple orb was a dark 'stain' that seemed to be naturally in the shape of the sigil for 'Earth,' or rather the name of the planet that Kain was currently on. Apparently, its true name is not Earth, the true pronunciation has been long lost and is quite difficult to pronounce for humans, and so it had always been called Earth by humans.
But the sigil symbolizing the planet still remains—after all, it is a necessary component for any teleportation arrays being layed down on this planet.
Therefore, Kain strongly believed that this was the core of the planet.
