In society circles a tiara is like a (way) fancier wedding band — a sign that the wearer is married. So for a royal bride, her wedding day will be the first time she wears a tiara. It is often her
something borrowed" - Diana wore the Spencer family tiara, the Duchess of Cambridge's was borrowed from the Queen, and Zara Phillips' was borrowed from her mother Princess Anne — however Sarah Ferguson was an exception, and her tiara was her "something new." This tiara is likely to be Princess Eugenie's "something borrowed" on her own wedding day.