The casting form is the cuttle fish bone that folks use for their Parrots to chew on to trimm their beaks. We went through a bunch with the kids doing some casting for trinkets for them. I cleaned out the pet shop stock of them. The patter you see on the sliver is from the striations of the bone layers. It reminds me of the resin layers of pine when you burn the finish. I made the sight mold thicker to allow for the removal of the striations. The Elvish smiths in Lord of the rings show the casting of the Rings of Power with cuttle fish molds. If its good enough for them I can do it for my restoration.