The Rare Orb is a mystical round Crystal Ball that holds the power to open the door to the Golden Temple. All eight of them are required to activate the doorway.
To collect rare orbs you must complete all levels in a world with all K-O-N-G Letters. Once you have completed each level with K-O-N-G, a new level will be revealed in the world. Completing the bonus level will earn you a Rare Orb. The new level that is obtained after getting all of the K-O-N-G letters in a world is a temple. Temples are extremely difficult. Rare Orbs are held in a shrine at the end of a temple. Don't get too excited about the size of a rare orb, though. They are only about the size of DK's head.