You got me really thinking Franck! I think that Princess Diana had only the amethyst suite which she wore with different colours, including the burgundy/maroon coloured dress.
One of the things influencing my idea is the pearl bracelet with the amethyst and diamond centre/clasp which we know Princess Diana wore with the amethysts.
For example
At the ballet 1985 wearing a burgundy/maroon dress with the earrings and bracelet.
1986 wearing a purple dress with the earrings and bracelet.
Wearing the earrings, the bracelet and a pearl and amethyst necklace with a burgundy coloured dress. Because of the colour reproduction the central stones look dark reddish.
For a number of reasons, I suspect that the Duchess of Cambridge's earrings do not have rubies. I also think that the central stones in Princess Diana's jewels are cabochons; in the Duchess' it looks to me as if the stones are faceted.
What do others think? And what stones could be in the Duchess of Cambridge's jewels?