The Brabant Laurel Wreath tiara was given as a wedding gift in 1999 to Mathilde d'Udekem d'Acoz (now Queen Mathilde of Belgium), by a group of Belgian aristocrats when she married Prince Philippe, Duke of Brabant (now King Philippe of Belgium).
It's an antique, made by English jewellers Hennel and Sons in 1912. It can be worn as both a necklace and a tiara.
Here is Queen Mathilde wearing it as a necklace in 2015 on a state visit to Poland:
Here's her daughter, Crown Princess Elisabeth, wearing it as a tiara: