That's the Maria Feodorovna Sapphire Bandeau Tiara
From The 10 Most Fabulous Royal Sapphire Tiaras of All Time:
In the style of the Grand Duchess Adelaide Tiara, Maria Feodorovna’s Sapphire Bandeau is grand but not too lavish, featuring a starburst pattern of diamond rays radiating from a central blue sapphire. Some say the tiara originally belonged to Maria Feodorovna, Princess Dagmar of Denmark, the wife of Alexander III, who was Emperor of Russia, King of Poland and Grand Prince of Finland from 1881–1894. The Empress had a vast collection of jewelry that dispersed after her death, so if the tiara belonged to her, perhaps that’s how it fell into the possession of Queen Mary, wife of King George V of the United Kingdom and the British Dominions, and Emperor of India, from 1910–1936. Mary left the tiara to Queen Elizabeth The Queen Mother, who often gave it to her daughter Princess Margaret to wear. While Queen Elizabeth has never worn this tiara in public, as part of the jewels owned personally by the monarchy of the United Kingdom, perhaps the new princess will delight us by wearing it.
More from the Royal Order of Splendor:https://orderofsplendor.b...ay-marie-feodorovnas.html
Wherever the tiara came from, it was next seen on Princess Margaret. (According to The Queen’s Jewels again, Queen Mary left it to Queen Elizabeth the Queen Mother, though she was never seen wearing it.) Princess Margaret wore the tiara only on a limited number of occasions in her early years. She also wore a brooch that appears to be a match for the centerpiece of the tiara. That brooch was worn through her later years.The sapphire brooch hasn’t been seen since Princess Margaret died. The tiara hasn’t been seen since she wore it. Ownership of either piece is unknown at this time. (There's even the possibility that they ended up in two separate collections.) Will they be reunited? Will the Sapphire Bandeau reappear? The waiting game will have to continue.
I hope the brooch was reunited with the tiara and is HM's possession. It would look lovely on Bea.