After looking at both of the pictures, I agree with you that the darker colored gemstones in the tiaras appear to be different in size but it seems strange to me that Queen Alexandra would have two tiaras of such similar design but with different gemstones. Perhaps the tiara started out with cabochon sapphires and she later had the tiara altered to use the larger amethysts thus removing the inner row of diamonds. This is complete speculation of course but since we know that the necklace the Countess of Southesk is wearing in the picture is made of amethysts, it makes since that her tiara would also be amethyst. According to Ursula's site, in 1946 Maud sold a sapphire and diamond collar, an amethyst and diamond tiara, and an amethyst and diamond necklace that all belonged to her grandmother so that would account for all three of the pieces of jewellery discussed but explain the difference in the tiara's appearance when worn by Queen Alexandra and the Countess of Southesk.