Which low temperature (< 350°C) solder can be used to solder a ring? So non toxic in direct constant contact with skin and overall in domestic environment?
Au/Sn solder should do it. Available at http://www.indium.com/products/
The eutectic point is 536 deg F, or 280 deg C. I can't vouch for the biocompatibility for permanent skin contact. The best source for that would likely be with the compliance officer for whatever standard you're trying to hit, or maybe a phone call to the Indium Corporation.