I often find these, what I assume to be shells, during walks along the beach. I haven’t seen any creature or plant alive that has this feature and research attempts haven’t been successful. There are bright reflective blue streaks along the shell and it is yellow-brown coloured and transparent. I have found these along the Northern Irish coast
I believe it's a Patella pellucida or blue-rayed limpet. They live off seaweed in deeper waters, so it's not a surprise that you wouldn't see many live ones.
Picture from an interesting article: Blue-rayed limpet holds secret for windscreen transparent displays with no internal light source