As Brad say, heat does not travel well through platinum. And if you
want to keep the diamonds cooler, you can use a paste (I don’t know
the English name) that prevents heat travel. You can also protect the
diamonds by rolling them with wet Kleenex.
For soldering silver you have to heat around the soldering point
because heat travels well through silver, but for soldering small heat
conductive alloys such as plutinum and gold you can solder only
heating the soldering point. So you should use very narrow torch for
soldering. And I recommend to use white gold for safety.