Q-Workshop Call of Cthulhu: Black & Glow in the Dark Dice Set - Rules of Play says:
Behold the Call of Cthulhu Black & glow-in-the-dark Dice Set (7). Right after the first throw, you will understand that you should care about aesthetics and never let the game change into a primitive d100 throwing and rattling. You can play using casual dice, but where's the style and elegance?
