Trilobites also used calcite (same mineral corals place down) lenses, which means that rocky complex eyes may well have evolved on Earth before squishy ones.FUN FACT: Rock eyes sound like something of fantasy? While GF probably weren't thinking of this animal, there is an animal which sort of does have rocks for eyes! The sea mollusk chiton has eye structures on its shell made out of aragonite. It seems to be able to tell the difference between sizes (and maybe shape) of shadows and differences in light, though probably not much else. The eyes being like this is an evolution compromise/trade-off as if the eyes were any bigger, complex, or made of more organic material it would likely have to shrink the size of its shell. So to keep the shell its eyes were evolved with a simple & specific function with the less intricate structure/material possible.
Man, we haven't had an invertebrate fossil mon since gen 3, despite how much potential there is.