This article is from the Piercing FAQ, by Anne Greenblatt with numerous contributions by others.
Gemstones can be set in metal using either a prong setting or bezel
setting. Bezel settings refer to a bezel cup or ball which holds a
gemstone. Both settings can be threaded for barbells or labret studs.
Bezel settings can be dimpled on either side to be worn as captive
Gold bezel settings are generally more secure than steel bezel
settings because gold can be shaped to overlap the edge of the stone.
Cheaply-made steel settings do not overlap the edge of the stone
because steel is more difficult to shape; the stones are usually held
in place with epoxy. The difference between the two types of bezel
settings is visible; the edge of the setting should overlap the edge
of the stone. Settings affixed with epoxy usually cannot be safely
autoclaved. Such settings should not be worn in tongue or genital
piercings because the epoxy can deteriorate and weaken when exposed to
saliva and urine.