The ley-stone is a pale replica of a true Key of Kings. Like the key itself, it serves as a tuning fork to merge the wielder's will with that of YISUN, but on a far weaker level. The King's Doors too are in fact, tuning forks. Ley-stones resemble small red gems, and it is possible but unlikely that a Key of Kings could be mistaken for a particularly bright ley-stone.
A ley-stone does not allow one to use The Art to teleport directly into the Void from a world, while a Key of Kings does. Any key enhances the natural ability of the wearer to impose their will upon the universe, so things like division do naturally become easier for them, allowing teleportation from the Void to Throne. Ley-stones amplify one's proficiency with the Art.