Funny, I had just been thinking about that iron-armored snail* and wondering if there were other organisms that incorporate iron or other metals into their shells/bone/teeth and if the metal provides structural benefits beyond what could be achieved without it (given how tough bone and shell and teeth can be). Although it may not (just) be about the material properties - sounds like the snail’s scales are also about removing sulfur compounds from the body.
*The scaly-foot snail, that is:
