Louise "Sr." (daughter of Queen Victoria) married the Duke of Argyll in 1871. Louise "Jr." (daughter of King Edward VII) married the Duke of Fife in 1889.
It would be interesting to know when exactly this emerald tiara was given away by the Queen. We know that, after Prince Albert's death in 1861, the widowed and ever-mourning Queen Victoria only wore pearl and/or diamond jewels with her black clothes. After 1861, Queen Victoria progressively handed out her coloured jewels to her daughters and daughters-in-law; but she probably retained some of them in her possession, and gave them only years or decades later, or even by bequest after her death in 1901. As far as I know until now, we just know that this emerald tiara went at some time to the Duchess of Fife, but we do not know when and for which occasion (wedding gift, bequest...). If your book can help to clarify this, that would be great!
It would be also interesting to know which inventory of the Queen's jewels is mentioned in the book about the Royal Archives. In The Queen's Diamonds (2012), Hugh Roberts mentions (page 43) two inventories of the jewels of Queen Victoria, one made in 1858 (and focusing in priority on the jewels considered as Crown property), the other in 1896 (far more comprehensive and recording the eventual disposition of each jewel).
--Previous Message-- : Arthur, : : This is definitely the tiara, so I do think : you have solved the mystery. I'll have to : verify when I get home the date given in the : book to confirm which Louise received it as : a wedding present. : : BTW, the book also indicates that the gift : was composed of the tiara as well as the : necklace and brooch. :