The Spencer Tiara is actually not a royal tiara. It belongs to the family of the late Diana Princess of Wales. She wore it so many times that people assumed it was a royal tiara.

Interestingly, the Spencer Tiara is made from pieces of different eras and different jewelers. The original pieces were thought to been from a tiara that belonged to Frances, the Vicountess Montagu that was passed down to Lady Sarah Spencer (1838-1919), who was the the daughter of the 4th Earl Spencer. Lady Sarah Spencer gave the center piece of the tiara as a wedding gift to Lady Cynthia Hamilton when she married Viscount Althorp, the future 7th Earl Spencer in 1919. Cynthia was Diana's grandmother and (really?) the Lady of the Bedchamber to Queen Elizabeth the Queen Mother. In the 1930s Garrard was commissioned to design four more pieces for this final version of the tiara.

The design in its final shape includes diamonds mounted in gold in various floral designs, stylized tulips, star shaped flowers and foliage scrolls.

It was worn by Diana's sisters, Jane and Sarah at their weddings and by Victoria Lockwood for her wedding to Diana's brother, Charles, the 9th Earl Spencer. 

Obviously, the most memorable appearance was for the royal wedding to Prince Charles in 1981. When Diana was the Princess of Wales, she wore the Spencer Tiara quite a few times. She had to borrow it from her father and then her brother and that was thought to have been a difficult ask. The Queen loaned her the Cambridge Lover's Knot Tiara, but it was heavier and have the moving parts of the swinging pearls. It was much more difficult to wear. these were the only two tiaras that Diana wore.

Significantly, Diana wore the Spencer family tiara as a show of pride of her family and you can't argue with that.