The topic is eye miniatures. Popular from 1780 through 1820, and made fashionable by George the IV's romance with an unsuitable widow, eye miniatures are portraits of your lover's eye. Just one eye.
Like an early, early form of texting penis photos (so glad I missed that trend), but way classier and this went both ways (lady eyes, dude eyes), these portraits were painted on ivory and typically incorporated into jewelry. I really think that we are not holding modern men accountable--can you imagine a guy (a) commissioning a portrait, and (b) having a jeweler set it? Maybe, as part of the current throwback trends, those bearded hipsters in Brooklyn are craftily romantic. Someone date one and report back.
There are definitely examples later than 1820 (my google image search showed one from 1905); later versions are probably ceramic or enamel. If you are a true collector, then you'll hold out for Georgian examples. If you are terrified of losing something of historical significance, you can go with the later versions or find someone on Etsy knocking them off. Yes, it's a little odd and maybe creepy. But when it comes to collectible antique jewelry its not nearly as weird and creepy as Victorian human hair jewelry and much, much harder to find.
That is some weird shit!
But aren't you glad you know it exists? (PS: If the pic doesn't have a link, I found it on this Pinterest collection. )