I have found 2 references to Queen Alexandra wearing a ruby tiara.
Both items came from The Times.
1. 18 July 1904, p. 14. "Ruby and diamond tiara. Ornaments - rubies and diamonds."
2. 23 May 1908, p. 15. Tiara, rubies and diamonds. Ornaments, rubies and diamonds."
During this period the paper never says a necklace, earrings etc, it simply refers to the Queen's ornaments - as opposed to her tiara. Then it lists the Orders she wore.
This tiara cannot be part of Queen Victoria's ruby parure as she gave that to Princess Beatrice as a wedding gift.
I am not aware of another ruby tiara owned by Queen Alexandra or Queen Victoria, so I wonder if this is what we call the Oriental circle tiara.
I also wonder if Queen Alexandra might have worn the Timur ruby necklace-- although I realise this is an assumption.