Diamonds of just about any carat appear best when set in platinum or white gold, which are both cool-toned and frosty metals. Except maybe white gold, which has a tendency to become a yellow tone over time, platinum retains its original color. Although platinum typically develops a patina over time, this may be avoided by taking it to a jeweller. And with something like a gorgeous diamond necklace that you'll only wear once in a while, it shouldn't be an issue.