I think with the right re-working, Kate would wear the Burmese Ruby tiara well one day. she very much suits jewel tones and rubies would look lovely on her.
Likewise Vic of Sweden would suit the Edward VII ruby tiara the SRF owns. It's a controversial piece but it deserves to be seen more.
I think the Brits should just take one of their million ruby necklaces and put it on a frame to make a tiara for Kate. Same with an Emerald necklace, although the Greville Emerald tiara would also look amazing on her. They have so many stunning pieces, they should issue each bride tiaras of all the colors.(except purple,since they have almost no amethysts. The Saudis should consider that before the next state visit. If they have to spend millions on jewelry, they should try to fill the gaps in the collection.) At least do so for Camilla and Kate. Sophie should also get more and better pieces, but that's just because I love her.