A vibrant amethyst cross favored by Diana, Princess of Wales, will headline Sotheby's "Royal and Noble” online auction from January 6 through 18. The piece carries a presale estimate of £80,000 to £120,000 ($96,000 to $144,000), but could sell for much more due to its royal provenance.

“Jewelry owned or worn by the late Princess Diana very rarely comes on to the market, especially a piece such as The Attallah Cross, which is so colorful, bold and distinctive,” said Kristian Spofforth, head of jewelry for Sotheby’s London.

According to the auction house, the late '80s was a time when Diana was making bolder fashion choices and taking more autonomy in her life. It was during this period, in 1987, that British Crown Jeweler Garrard lent her an eye-catching amethyst-and-diamond cross to pair with an exquisite baroque-style purple and black velvet dress by Catherine Walker & Co.

She wore the ensemble to a high-profile event held at Garrard's London headquarters in support of Birthright, a charity which strives to protect human rights during pregnancy and childbirth. Diana confidently dangled the large 136 x 95mm amethyst cross from a waist-length strand of pearls.

Designed by British Crown Jeweler Garrard circa 1920, the piece was originally commissioned by a regular customer, but later circled back to Garrard, where it was purchased during the 1980s by the late Naim Attallah (1931-2021), a high-level executive at the jewelry company.

Over the years, she would continue to collaborate with Garrard, borrowing her favorite amethyst and diamond cross on many more occasions, as recalled by Naim Attallah's son, Ramsay.

“Princess Diana and my father were friends, and I remember that she often came to see him at the historic Garrard store on Regent Street, where his office was, and she would ask to borrow the pendant on several occasions. She really loved the piece,” Ramsay related via Sotheby’s.

According to Ramsay, nobody other than Princess Diana has ever worn what is now known as "The Attallah Cross."

“When I was growing up, we’d always have it on the table for Christmas lunch," Ramsay told vogue.com, "but it was never worn by anyone other than Diana and it hasn’t been seen in public since she died [in 1997].”

Designed with square-cut amethysts and circular-cut diamonds, the fleurée cross has a total diamond weight of 5.25 carats.

Fans of Princess Diana may remember that her famous sapphire-and-diamond engagement ring was sourced at Garrard.

Back in February of 1981, Prince Charles proposed to the 20-year-old Lady Diana with a ring that the future princess picked out herself. According to the editors of Vogue, some members of the British royal family fumed at Diana’s choice — not because it featured an unconventional center stone, but because it was a stock item from the Garrard catalog.

Founded in London in 1735, Garrard was the official crown jeweler of the UK from 1843 until 2007. The distinguished company that had been entrusted with the upkeep of the British Crown Jewels was the logical source for Diana’s bridal jewelry.

So, in the lead-up to their engagement, the 32-year-old Prince Charles presented his bride-to-be with a bunch of design options from Garrard. Her favorite was an 18-karat white gold ring set with a 12-carat oval Ceylon sapphire surrounded by a halo of 14 round white diamonds.

In Diana’s eyes, the ring was perfect. She loved it so much that she didn’t request any modifications or customizations.

In the eyes of her critics and some members of the royal family, the ring was sub-standard because it was hardly unique. Critics called the Garrard stock item a “commoner’s ring” because any non-royal with a $60,000 budget could purchase the exact piece.

Nevertheless, Diana’s sapphire and diamond engagement ring would become one of the most recognizable and imitated engagement rings of all time.

Credits: Amethyst cross photos courtesy of Sotheby's. Photo of Princess Diana dancing with John Travolta at the White House in 1985 by United States Federal Government, Public domain, via Wikimedia Commons.