Note the smooth gemstone mounted on the lid of this keepsake box. The art of cutting stones with angular facets didn't really catch on in Europe until the 14th century or so. A DM could use this to his or her advantage by making normal gemstones appearing in treasure the smooth, polished type pictured above, but still using cut stones as rarer, more expensive gems. Call them "dwarf-cut stones" or designate them as being from a faraway land and Bob's your uncle.
The design of this ring intrigues me. The stone is smooth, for the reasons mentioned above. But the barrel like setting looks like it could unscrew to reveal a tiny compartment. Rings with compartments aren't a new idea. They traditionally carry a dose of poison to be slipped into someone's wine. But there are other options in D&D, such as a pinch of dust of sneezing and choking, a scroll or map penned by a tiny person like a pixie, or even a supernatural entity bound inside like a smaller, less obvious version of a genie's bottle.
Not an undead or mutant, just a poor bastard with a bad case of leprosy. I could see a jumpy PC accidentally murdering this sorry fellow. Hell, a low level magic-user with charm person could frame the PCs by asking their charmed leper friends to act like zombies. A higher level might use actual zombie lepers as a way of doubly discouraging PC interference.