For her first official visit to the U.K., the Dutch monarch Queen Maxima of the Netherlands appropriately pulled out all of the stops. While she brought along her husband King Willem-Alexander as her date, her most notable companion was by far the Stuart tiara—home to the Stuart Diamond—which ended up being the star of the ballroom at Buckingham Palace during her royal banquet with Queen Elizabeth II, Prince Charles, and Kate Middleton.
This wasn't the Stuart Diamond's first trip to the U.K.: Queen Juliana also wore it on her state visit in 1972. If you're wondering why it's caused such a stir, the diamond has quite the history—starting with Queen Mary II's order that it not be cut into two diamonds, as was common practice at the time, but maintain its full size, when she and her husband King William III purchased it in 1690.
Alas, the diamond would only be in Queen Mary II's possession for a few years: She died from smallpox in 1694, and upon her husband's death in 1702, it was returned to the Netherlands, as it was thought to once be the property of the House of Orange. (Despite Mary's sister Anne suing in protest.) It wasn't long, though, before it returned to England, when the French army invaded the country, leading its owner, Prince Willem V, to seek asylum in the U.K., where he and his wife were informed that it was 39.75 carats. Upon their return home, the diamond was removed from its then position on a necklace and later transferred to the tiara, which King Willem-Alexander's great grandmother, Queen Wilhelmina, wore for her coronation in 1898.
Now that the tiara is in the hands of the Queen of the Netherlands—who is the Netherlands' first queen consort to have been born as a commoner, as well as outside of Europe—Queen Maxima has been putting it to good use; she also wore it this past May, on a visit to Luxembourg. (Though that's not to say she's forgotten the rest of her extensive collection: Later that month, she wore the Dutch Diamond Bandeau, also notable for its size, to Crown Prince Frederik's 50th birthday gala in Denmark.)
While she's earned a reputation for her style—and particularly her hat collection—Queen Maxima's original entry into the public eye was met with much controversy. Born in Buenos Aires in 1971, she met Willem-Alexander in 1999, who introduced himself simply as Alexander, leading her to believe it was a joke when he later informed him that he was a prince and heir apparent to the throne. They married in 2002—though only after the Dutch parliament investigated Maxima's father Jorge Zorreguieta's reported involvement in Argentina's so-called "Dirty War," a military junta that led more than 30,000 people to disappear.
Things have of course lightened up for Queen Maxima since, though she did face some tiara-related competition at the state banquet. Kate Middleton also made headlines for borrowing and wearing one of Queen Elizabeth II's tiaras, bearing a marked resemblance to her late mother-in-law Princess Diana by completing her look with a pale blue dress.