Always match the solder to the color and karat of the metal you are soldering. Do not use low karat “repair” solder or solders of lower karat then the metal as the solder may tarnish, causing the jewelry to look dark around the solder joint. This is particularly important when attaching crowns to rings. If low karat solder is used the ring may turn un-slightly dark around the base of the crown. The only remedy to this problem is to remove the crown and clean away the old solder. Then solder a new crown to the ring using the correct solder and reset the stone.