Conversion Gel is a white gel that enables any surface to have portals placed on them, even if they are not normally portal-conductive.
According to Cave Johnson, conversion gel is made from ground-up moon rocks, which make a great portal conductor. This is shown when Chell fires a portal at the moon near the end of the game, despite the non-flat surface of the moon. Unfortunately, long-term exposure to ground up moon rocks led to Johnson's poisoning and eventual presumed death. He attempted to fashion a cure by forging a gel out of the rocks, in hopes it would "leech the lunar poison out of a man's bloodstream."