Princess Diana chose to wear a tiara from her maiden family's collections, the Spencer Tiara, a heirloom of the Spencer family. The tiara is closely associated with Late Princess Diana even though it has been worn by a few Spencer ladies over time.
The Spencer tiara has beautiful curved floral motifs studded with diamonds. In contrast with normal Royal Tiaras the Spencer tiara is a combination of several elements. Two of the elements are said to have come from a tiara owned by Frances, Viscountess Montagu and left to Lady Sarah Spencer in 1875.
The central element was a wedding gift from Lady Sarah Spencer to Cynthia, Viscountess Althorp (Diana's grandmother) in 1919. In the 20th century, it was remounted and four other elements were made to match it.
Princess Diana wore the Spencer tiara to a lot of formal functions, parties & banquets, maybe because it is lighter in weight as compared to the Lover's Knot Tiara gifted to Princess Diana by the Queen Elizabeth.