The Soul Gem
The Soul Gem is one of the Four Universal Jewels (the others being the Karma Gem, the Heart Gem, and the Mind Gem). Combined, these four gems form The Unity Stone, which (legend has it) bestows god-like powers to the possessor. The Chosen One is prophesied to combine the gems and guide the unification of all the planes of existence into true, lasting Oneness.
The Soul Gem has a soft, light blue hue. Its powers are unknown; legend ascribes to it everything from the power to travel to various planes of existence, speaking with the dead, and even resurrecting loved ones.
The four Gems were scattered a thousand years ago, after a mad cleric managed to combine them briefly and threatened all of existence. Its current whereabouts are unknown.