Ryll kor represented the most scarce and valuable classification of ryll, accounting for only about three percent of the total ryll supply. It had the characteristic of absorbing light across the spectrum, with the exception of ultraviolet wavelengths, leading to a charcoal black visual appearance, unless observed by individuals capable of perceiving ultraviolet light.
To combat the Krytos virus outbreak that affected the inhabitants of Coruscant, Ryll kor was utilized in conjunction with bacta to formulate rylca, which served as a remedy for the viral infection.