Ryll kor was the rarest grade of ryll. It made up approximately three percent of all ryll. Ryll kor absorbs light except in the ultraviolet range which gave it an appearance of charcoal black except to someone who can see in ultraviolet range. When the Coruscant population was afflicted with the Krytos virus, Ryll kor was combined with bacta to create rylca as a cure for the virus.