Guests at coronations tend to wear white jewellery unless their coloured jewellery is much grander. If the Countess of Southesk owned the amethysts, the emeralds and sapphire necklace and she chose to wear the amethysts that would be an odd decision. She was not attending as a peeress so avoiding a colour that would have clashed with her red velvet robes would not have been an issue. Emeralds are are precious stone whereas amethysts are not. She might have been trying to honour her grandmother and thereby remind people of her link to her, but if she owned the emeralds that would have done the same thing.

It would have been reasonable for the Duchess of Fife to choose not to wear emeralds since she had the Fife Massin Tiara or even her Russian Fringe.

My guess is that Alexandra Fife got the lions share of the inheritance since she was the senior heir and that Maud Southesk was regarded as a younger daughter who received good items but not major pieces. Perhaps her main items were the sapphire collar and the amethyst suite. Alexandra Fife got Mar Lodge which seems to have been the main Fife residence at the time. The family gave Duff house to the people of Banff in 1903 well before the Farquhar affair so they seem to be a lot less wealthy than people supposed.