The renowned British jewelry designer Theo Fennell has created a series of four elaborate one-of-a-kind rings featuring hidden compartments, reminiscent of the famous 16th-century European poison rings that concealed a toxic substance to be used on one’s enemies. (Later these small treasure boxes held locks of hair or miniature portraits.) Fennell’s Opening Rings puts his own artistic vision on this age-old concept. The designer collaborated with the sculptor Willard Wiggan to create the Empty Quarter ring ($2…