It's one of the most recognisable tiaras for modern royal watchers. Most closely associated with Princess Diana, it began its life in 1913 when Queen Mary asked Garrard & Co. to make a copy of a tiara owned by her grandmother. After Queen Mary's death, it passed down to Queen Elizabeth, who gave it as a wedding gift to Princess Diana.
It returned to the royal collection following Diana's death, and it's most recently been seen being worn by her daughter-in-law, the Duchess of Cambridge. The exact value has never been confirmed, but estimates are typically in the £700,000 to £2 million range.