Apparently there was a trend — beginning in the late 18th century, and continuing for a few decades — of lovers exchanging tiny portraits of one of their eyes, and wearing them mounted on jewelry.
While miniature portraits were already popular in eighteenth-century England, they were often private objects viewed solely by the wearer. Yet an eye portrait could be worn boldly on a bracelet, ring, stickpin, pendant, or brooch, with the identity of the subject a mystery.
Similar to exchanging locks of hair, the eye portraits helped keep a person close, even when separated by distance or the decorum of Georgian courtship, which limited public romantic gestures. They also channeled a desire to be seen. Art historian Marcia Pointon explores this context in The Art Bulletin, noting that the "word gaze in this period denotes a fixity of looking or staring that implies a degree of self-consciousness on the part of the looker and the looked at." So the eye miniatures are not only standing in for an absent person like a miniature portrait would, they're evoking that charged act of looking. Painted with incredible detail in watercolor on ivory, they also evoked a vigilant stare to remind the wearer to be faithful while their beloved was away.
It's rare to find a trend that so effortlessly shifts from "beautiful" to "creepy" depending on what angle you hold it.
There's something really modern about this, too — the creative and kooky use of media, that little jab of surveillance culture, the blend of anonymity with public-ness, the webcam-like nature of the painted eye hanging around your neck. I could totally see this trend coming back, with the eyes being painted, or even animated gifs of your lover's actual eye blinking on a tiny LCD panel on a chain.
Go check out the JSTOR article — it has reprints of several more of the eyes, which are quite mesmerizing.