What you have is a composite material that looks similar to mother-of-pearl. It is smooth to the touch like other plastic keys and is very common on piano-accordion keys.
I have seen that material used on the so called (In France) "English piano" of some old era, but not on the keys, as a decor on the case (strips, litle squares, incrustations) .
BTW do you have an idea of the undercoat used on those varnished pianos (often very shiny, and alittle thick, I heard of plaster or skin glue used to close the grain)
The material on the accordion is possibly celluloid dont you think ?