Some exhaust valve seats are made of Stellite, a proprietary alloy with about 10% tungsten content according to the link. Judging by the hardness it looks difficult to machine. Probably makes a good bezel front if the machinist has access to grinding equipment, or can design the light so the valve seats press in.
I suppose you could design a light made out of a stack of valve seats. You'd have really good control over the contour if the seats come in enough different ODs.
I'm going to move this into MMM for you, as I think that is a better place for it. I'll leave behind a redirect to it. But no sales or feelers please - if and when you want to sell these, please post in B/S/T.