In December 2013, Kate wore her second-ever tiara when she attended an official event at Buckingham Palace while wearing the Lotus tiara. Like most of Kate's borrowed jewels, the item originally belonged to the Queen Mother. The tiara was originally a necklace and was given to the Queen Mother as a wedding present from King George VI in 1923. George later had the item dismantled and worked into a tiara by Garrard. (This practice is actually pretty common among the royals, and Diana was famous for reworking royal jewels in interesting ways.) The tiara was eventually given to Princess Margaret, who wore it to many official events. It was also worn by Serena Stanhope when she married Margaret's son, Viscount Linley, in 1993. After Margaret passed in 2002, the tiara somehow ended up back in the queen's main collection and was loaned out to Kate.
Source: Getty / Kypros, Splash News Online