There's quite a few iridescent things in the sea. I have a couple of cabinets filled with different kinds of shells, since the sea is nearby where I live. One cabinet is filled with domestic shells, the other is filled with fossils and foreign shells. In the second cabinet there's a stack abalone shells (I think) of different sizes. It's great that this theme fell on a 'full drawing' day, so I could use this as a bit of a reference study.