One of my all time favourite tiaras is the Russian diadem below. https://www.instagram.com/p/Bno4Qk1hNlD/
I recently joined an Instagram page titled Russian Treasure https://www.instagram.com/russian_treasure/
It included a post on this very tiara and it states the following:
"A masterpiece of famous Russian court jeweler J. Duval, tiara with diamond pendeloques and large heart-shaped gem in the center, has long been in the possession of the Württemberg Royal Family. Originally commissioned by the Dowager Empress Maria Feodorovna in 1803, the tiara was evidently lent to her daughter-in-law, Empress Elizabeth Alekseevna, as this tiara is depicted in the portrait of 1828 and on one of the miniatures in the hairstyle of the consort of Alexander I. A few years after the death of Maria Feodorovna the jewel could become part of the dowry of one of the Grand Duchesses, since the inventories of the different years do not mention the presence of the tiara with pendeloques in the collection of Russian Crown Jewels. In the official portraits of Empress Alexandra Feodorovna of 1911 again found a recognizable tiara, which three years before the publication of pictures, was bought from the Duchess Vera Konstantinovna of Württemberg, great granddaughter of Maria Feodorovna. Traces of the tiara with pendeloques lost after drawing up of the Soviet catalogue of the Russian Crown Jewels of 1922."
Adding 2 and 2 two together, I can only assume that the tiara formed part of the dowry of Olga Nikolaevna of Russia, daughter of Tsar Nicholas I of Russia and granddaughter of Tsarina Maria Feodrovna of Russia.
She became Queen of Württemberg, however was childless. However she did "adopt" her niece, Grand Duchess Vera, who married a member of the Wurttenberg royal house and who it appears owned the tiara before it was sold back to the Russian Treasury in around 1908/09.
Vera's motivation for selling it is unclear as she was considered to be fabulously wealthy, being the heiress of her "adopted" parents Charles I and Olga of Württemberg.
I discounted that it could have come from Catherine Pavlovna of Russia, Tsarina Maria Feodrovna of Russia's daughter and who was briefly Queen of Württemberg as she produced two daughters, one of who became Queen of the Netherlands, who likely would have inherited her jewels rather than being kept by the Württemberg treasury.
I havent been able to find any evidence of either Queen Olga or Grand Duchess Vera wearing it though, in all of the pictures where she is wearing a tiara, most likely for events associated with 1896 Coronation, she is wearing an impressive Kokoshink fringe tiara, along with a sizeable brooch and some chunky emeralds. http://www.gogmsite.net/_Media/1896-coronation-ball-celebr-2.jpeg