Prince Rainier, Grace Kelly, and her parents pose during a press conference announcing Grace and Rainier’s engagement, January 1956 (Pictorial Press Ltd/Alamy)

As Monaco celebrates its National Day today, I’ve got a closer look at two of the most intriguing jewels from the late Princess Grace’s collection: her engagement rings.

Grace shows her engagement ring to her mother, Margaret Kelly, during the engagement press conference, January 1956 (AFP via Getty Images)

Yes, you read that right: rings, plural. The first, more modest ring was presented to Grace by Prince Rainier when he proposed to her over the Christmas holidays in December 1955/January 1956. The actress and the prince had met the previous May, and they’d carried out a sort of epistolary romance, corresponding across the Atlantic while she filmed movies and he dealt with drama over the insecure succession in Monaco.

Grace shows off her ruby and diamond ring during the press conference in Philadelphia held to officially announce her engagement in January 1956 (Pictorial Press Ltd/Alamy)

A few days after their second in-person meeting, which happened at Christmas in 1955, Rainier offered Grace a diamond and ruby ring from Cartier. The public got a first glimpse at the ring on January 5, 1956, when the engagement was officially announced at a Philadelphia country club. The Philadelphia Inquirer described Grace’s ring during the official press conference: “Grace, wearing a champagne colored dress of wool with gold metallic dots and gold collar clips and earrings, sported a handsome engagement ring, which the Prince had ordered fashioned from two family heirlooms, in the form of a diamond circlet and ruby circlet intertwined.”

Grace wears her second engagement ring in a scene from High Society with Frank Sinatra

The diamond and ruby engagement ring was described prominently in news reports in the day following the engagement, but when Grace returned to Hollywood to begin filming High Society, another ring suddenly appeared on her finger instead. Press shots from the set on the first days of filming in January 1956 show her wearing an enormous diamond ring while in costume for her role as Tracy Lord.

Grace “polishes” her diamond engagement ring in a scene from High Society

The press picked up on the ring swap, and Grace addressed the switch in a press interview. She told gossip columnist Louella Parsons that she’d been due to wear an prop diamond ring in High Society, but Rainier had offered her a real one as a second engagement ring. She happily explained, “now I can wear my real engagement ring” in the film, after getting the okay from the film’s director. The Philadelphia Inquirer trumpeted that the ring would be “one of the ‘stars’ of the picture.” One High Society scene, which playfully acknowledged the ring (and, implicitly, the press surrounding Grace during filming), even shows Grace breathing on the ring and polishing it on a pillow.

Grace’s diamond ring, on display with other Cartier jewels at the Denver Art Museum in 2015

The second ring was a stunner. Also made by Cartier, it featured a 10.5-carat emerald-cut diamond flanked by a pair of diamond baguettes. (Press articles from 1956 exaggerated the size a bit, calling it a 12-carat diamond.) So why the switch? Grace’s friend Judith Balaban Quine wrote that the original ring had always been intended as a “friendship ring,” a placeholder while a “proper engagement ring” was being made.

Grace wears her diamond engagement ring in a portrait, 1956 (ScreenProd/Photononstop/Alamy)

Quine also revealed that Grace always knew a bigger ring was coming, and she even suggests that the prop that had been made for High Society served as the inspiration for the real diamond ring Rainier commissioned for Grace. But why hadn’t Rainier simply brought along a traditional diamond engagement ring to America? Just before he sailed back to Europe in March 1956, he told gathered reporters simply that “it is silly to count your chickens before they are hatched.” In the end, both the ruby and diamond friendship ring and the massive diamond engagement ring became regular parts of Grace’s jewelry wardrobe, worn until her death in 1982.