The Precioustone is the focal point of Issue 251 story, The Universe Game. The gem is the prize of said game between Lumina Flowlight and The Void, where The Void plans to use it to gain ultimate power.
Little is known about the stone itself, but it has parallels with the Chaos Emeralds in that it will grant the holder limitless power. It is unknown if this is definitely the case, or how it works, but it is precious enough to be the subject of a hundred year war.
In Sonic Shuffle, the heroes are summoned to repair the Precioustone and defeat The Void.