Heat does not travel well through platinum. If the diamonds only go
half way around the ring, you should have no problems fusing the
shank or using 1700 solder.
For soldering near the diamond such as adding a crown, all platinum
solders have too high a melting point. You will need to use white
gold solder. I recommend 19k or 20k white weld. Make certain you
have a tight fit and use as little solder as possible.