Oh, those Royals.  They're always starting a trend.

In 1784, Britain's young Prince of Wales (future King George IV) began a clandestine relationship with Maria Anne Fitzherbert, a Catholic widow, whom he was forbidden from seeing, both by law and by his father, King George III.  Nevertheless, he pursued her.

Mrs. Fitzherbert rejected the Prince's advances and left for the Continent in order to escape his ardent attention.  The young prince faked a suicide attempt to show his great despair, they exchanged letters and Mrs. Fitzherbert's heart softened, blah, blah, blah.  She accepted his proposal of marriage then broke it off, more letter were exchanged, another proposal was accepted but instead of sending her an engagement ring he sent her an intimate small portrait of one of his eyes.  

Thus began the Georgian fad of painted eye broaches.

Source

Dubbed Lovers' Eyes, the optical art was often enhanced by a tear
to convey how much the loved one missed their intended.

It became a game in lovers' social circles to guess the identity
of the (often secret) admirer just by the painting of their eye.

Broaches weren't the only settings for the eye portraits - lockets, rings, small
frames and jeweled animal pins were also used to hold the unusual art. 

Because the tiny portraits were often discarded after the affair was
over, finding an authentic Georgian eye broach is near to impossible.
Today most of them reside in private collections or in museums.
It's estimated that less than 1,000 still exist.

Link

Oh, and the Prince and Mrs. Fitzherbert affair?  She returned to England and they secretly married on December 15, 1785, at her London residence.  Shortly after their marriage she commissioned a tiny portrait of her eye as a gift to her new husband.

Sadly, the Prince's father never gave his permission for the lovers to wed so the marriage was considered null and void.  In 1795 the Prince married his cousin, Princess Caroline of Brunswick but the two were completely incompatible.  Following the birth of their only child, Princess Charlotte, they separated and were estranged until their deaths.  Although he had many mistresses the Prince remained attached to Mrs. Fitzherbert the rest of his life.

King George IV and the love of his life, Maria Anne Fitzherbert

Imagine if reality TV had been around during their time!