A fad among the elite in 18th and early 19th centuries was to commission a painting, around an inch wide, of their loved one's eye. These paintings were set in jewelry, such as brooches, lockets or rings, often surrounded by gems. They were done in watercolor on ivory, and occasionally tiny precious stones were added to the corner of the eye to look like glittering tears.
Supposedly, the first miniature eye portrait was commissioned by George IV, who wanted a token of his Catholic lover Maria Fitzherbert. They could never have a valid marriage under the Royal Marriages Act due to Maria's religion, so the Prince of Wales chose this one small body part in order to keep anonymity.
Today, these paintings are extremely rare and valuable, but there are a good deal of forgeries floating around.