Has anyone ever experienced a piano with key tops made of mother of pearl? Mother of pearl is beautiful, but would it impede playing?

I would say the biggest impediment would be that it's hard to get long flat lengths of mother of pearl. It would have to be done in pieces and that would get both ugly and expensive - would probably fall off eventually too. Actually, I don't think it would be so nice - too busy and garish. Mother of pearl is meant to be a point of interest, not the whole deal.

Sure. They exist. Do a Google image search.

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.

There may be a modern synthetic, but I have seen old squares with the real thing. It is a cold material, and I would guess that it would get slick with perspiration. I'm told the artificial material is called mother of toilet seat.

That's just funny .

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 ?