Bona Calvi Jewels are made in "brone" a special brass bronze alloy. Sometimes (and also depending on you ph) a green halo appears but is in no way harmful to the wearer. It can happen especially when the ring is new and can therefore have processing residues. We therefore advise you to clean and polish it now that is new and from time to time in the future.
Also we advice not wearing it when washing your hands. You can clean it with a jewelry cleaning solution. These precautions should be sufficient to avoid the green halo in most cases, but another tip to avoid the problem for good is to apply in the internal part of the ring a thin layer of clear nail polish.