Each Immortal has a specific purity: Rough, Polished, Piece of Art, Masterpiece.
The purer an Immortal is, the rarer it is and the higher its attached rewards. Immortal purity increases both the reward associated with the Immortal quest and the base reward earned when winning the game.Here is the scaling factor you can expect between all Purity levels:
As an example, this means that if the quest reward for a specific quest was 90 IP for a Rough piece, it would be 180 for the Polished version of it, 360 for the Piece of Art version, and 720 for the Masterpiece.