inlay material

What materials are used for the bird inlays on PRS guitars? Ignoring the high-end models of the Core series, why do some SE guitars use shiny shell inlays, while the us-made more expensive models like S2 and even the Silver Sky model only use acrylic for the normal bird inlays?🧐

I am in the process of ordering a Private Stock (build can be seen here) but I am not 100% sure on the inlay material. The fretboard and headstock veneer will be dark colored ebony. I know I want the hollow/outline "J-birds" and the Private Stock eagle headstock. I have gone back and forth...