"Also, I do will and appoint ten rings of gold to be made of the value of twenty shillings a piece sterling with a death's-head upon some of them."— a will dated 1648, quoted in Finger Ring Lore by William Jones
Rings have long symbolized the eternal -- the unbroken circle. "Memento mori" is a Latin phrase that literally means "remember you shall die," and refers to objects that serve as a reminder that death awaits us all, and for those who believe in an afterlife, that it, too, awaits.
Memento mori rings have been worn since ancient times. Some of the most elaborate were made in 16-19th centuries in England, but they were also worn in many other countries.