Some Proton discs (mostly seen on Eclipse) have a more opaque core that has pearly swirls in it, its a little more firm and grippy than regular proton - I have a couple Volts and an Amp that looks like this. I've just heard from others they call it FX plastic, but I guess its just a blend of Proton - pearly more opaque, grippy. They are out there, from my experience they are on the more Stable-Overstable side of things.
like the OG bronze teslas