From an engraving in a volume from 1871 entitled "Rambles of an Archaeologist Among Old Books and in Old Places".
"These death’s-head rings were very commonly worn by the middle classes in the latter part of the sixteenth and the early part of the seventeenth centuries... Luther used to wear a gold ring, with a small death’s head in enamel, and these words, “Mori sæpe cogita” (Think oft on death); round the setting was engraved “O mors, ero mors tua” (Death, I will be thy death). This ring is preserved at Dresden. Shakspere, in his Love’s Labour’s Lost (Act V. scene 2), makes his jesting courtier, Biron, compare the countenance of Holophernes to “a death’s face in a ring.” We have already adverted to a similar ring worn by one of Shakspere’s fellow townsmen.
In the “Recueil des Ouvrages d’Orfeverie,” by Gilles l’Egaré, published in the early part of the reign of Louis XIV., is an unusually good design for one of these rings, which we copy [here]. It is entirely composed of mortuary emblems, on a ground of black enamel."
This file contains ladies ring sizes 4, 5 and 6 in order. Each engraving is slightly different.