A personal letter from Princess Diana (1961-1997) with the child signatures of Prince William (40) and Prince Harry (38) is said to be auctioned for an estimated 18,000 pounds (about 20,000 euros). With the handwritten note, Diana apparently thanked a police officer for making a motorcycle show possible for William's birthday party.

The sons of the Princess, who died in 1997, and the current King Charles III. (74) are said to have been four and seven years old at the time. The letter, which is being offered by auction house William George, is dated June 1989 and is written on Kensington Palace stationery.

In addition to the personal letter, a piece of jewelry worn by Princess Diana will soon be auctioned off. The Attallah cross takes center stage at Sotheby's annual Royal and Noble sale in London. The jeweled piece of jewelery was reportedly worn on a necklace by Diana on several occasions. The cross belonged to the former boss of jeweler Asprey

"Jewelry owned or worn by the late Princess Diana is very rare to come onto the market, especially a piece like the Attallah Cross, which is so colourful, bold and distinctive," said Kristian Spofforth of Sotheby's London, according to Forbes ". "In a way, this unusual pendant symbolizes the princess' growing confidence in her choice of clothing and jewelry at a special moment in her life."