Poison ring

A poison ring or pillbox ring is a type of ring with a container under the bezel or inside the bezel itself which could be used to hold poison or another substance; they became popular in Europe during the sixteenth century. The poison ring was used either to slip poison into an enemy's food or drink, or to facilitate the suicide of the wearer in order to preclude capture or torture. 
In Italy to this day pouring someone a drink whilst holding the bottle with the back of the hand facing downward, so as to let something drop from a ring bezel, is called "versare alla traditora" (“traitor’s way pouring”) and still considered offensive.
Rings like this have been used throughout history to carry perfume, locks of hair, devotional relics, messages and other keepsakes, so they have also been known by other names. Artists would paint tiny portraits of loved ones, to be carried in what was called a “locket ring,” which was popular during the Renaissance. By the 17th century, jewelers were creating locket rings in the shape of caskets which served as mementos for mourners. These were called “funeral rings.” Rings with compartments are also called “box” rings or “socket” rings.
It is rumored that Lucrezia Borgia used a ring of this description. 
The origin of poison rings.
According to Marcy Waldie, who wrote about poison rings in the October 2001 article "A Ring to Die For: Poison Rings Hold Centuries of Secrets", published in "Antiques & Collecting Magazine", this type of jewelry originated in ancient days of the Far East and India. It replaced the practice of wearing keepsakes and other items in pouches around the neck. The wearing of vessel rings was so practical that it spread to other parts of Asia, the Middle East and the Mediterranean before reaching Western Europe in the Middle Ages. By then the rings were part of the “holy relic trade.”