The ring showcases a cushion-shaped cut of the gemstone "tourmaline," encrusted in diamond. The engraved mount is embellished with a mitre motif. It is said to date from the 1870s, but obviously could be older. This type of stone has been popular historically with bishops - it is the color of hope and also the color of the episcopal dignity (the prie-dieu of bishops has traditionally been draped in green).
Some bishops today still wear a gem set ring. Others prefer a more simple gold band. Rings were generally returned upon the death of the prelate, given to another bishop, or a member of his household or curia, or diocese or successor. Most episcopal rings were made of sapphire, amethyst, ruby, or tourmaline. Rings today often become the property of a diocese, remaining in their care and trust. Very rarely were rings of this quality interred with a deceased prelate.
The large size of the ring is reflective of the fact that it was to be worn with ease over the glove during pontifical rites with full ceremonial regalia. Sometimes such rings had an additional band to enable it to be also worn without gloves. It would be nice to see these rings now in private collections eventually purchased by well-meaning Catholics who can return them to Catholic hands or lend them for rightful use in liturgical rites.
Source: Sotheby's