Ancient trilobites had eyes made of crystals

Fun fact: trilobites were able to see thanks to eyes made of calcite instead of soft tissue. YouTuber Thunderf00t shows off a cool fossil and explains the phenomenon.

Interestingly though the trilobites eyes were made of stone, calcite to be exact, so there are many examples of fossilised trilobite eyes. You can get a trilobite fossil with amazing eye detail on ebay for about 50USD. From looking around, the trilobites from Morocco seemed the best to me. It was long thought that the calcite eye disappeared with the trilobites until it was recently discovered that brittle stars have many calcite 'eyes' all over their exoskeleton. Although it should be said theses 'eyes' are tiny!

Los Angeles County's Natural History Museum recently posted an interesting piece on how brittle stars "see" things.

Trilobites: the creature that looked on the world through STONE eyes! (YouTube / Thunderf00t)