Ah! So the same as the rest of their hard parts, I suppose. I suspected as much, but couldn't seem to find any paper that explicitly stated this.
Thank you!
In regards to use of calcite vs. chitin: doing a quick search: https://www.ualberta.ca/en/earth-sciences/facilities/collections-and-museums/treasury-of-trilobites/index.html
Were trilobites also unique in using calcite in addition to chitin in their exoskeleton? Do any extant arthropods use calcite in any significant way?
What I find interesting, then, is what advantage the trilobites may have gained by using a basic mineral for the lenses vs. organic chitin. Chitin must have a transparent form in order to function for the eyes in modern creatures? Hmm.
I read in one paper that trilobites may have actually formed some kind of dual-layer in their lenses to compensate for the double-refraction property of calcite.