Mourning pieces contained the eye of a departed loved one, sometimes set in a frame of pearls which symbolized tears. Larger pieces can be expensive. In the 18th century, if you couldn't afford ivory, you could commission less expensive paintings on vellum, a paper thicker than parchment. Today, however, paper miniatures are no less valuable than their ivory counterparts. Rings are often worth more than broaches because they are rarer.
Larger pieces can be extraordinarily costly.
Fine detail in the eye also increases value. Good documentation is needed to prove the origin of the jewelry. Where values climb, fakes follow. The example shown here is a cheap imitation using a computer image copied from a book. The miniature contains pixels rather than brush strokes.