When the Princess of Wales surprised the world with her ruby tiara

Tonight, the Princess of Wales is expected to join Charles III and Queen Camilla in welcoming the Sultan of Oman to a lavish state banquet.

The white tie dinner, which will be held at Windsor Castle as Buckingham Palace continues to undergo extensive renovations, will be the centrepiece of His Majesty Sultan Haitham bin Tarik's three-day state visit to Britain. Attended by senior members of the royal family, as well as politicians, diplomats, and other notable public figures, state banquets are among the few occasions when royal watchers are given a glimpse princesses, duchesses and aristocrats adorned in twinkling tiaras. And arguably none is more glamorous than the Princess of Wales.

Although there are dozens of gem-encrusted creations held between the royal and private collections, Catherine has only stepped out in five over the last 15 years. She favours one above all others: the Lover's Knot Tiara, which she has worn to seven of the 10 state banquets she has attended. The Princess choosing to wear any other tiara represents a major break from the norm – and none was more surprising than the tiara she wore to the German state banquet almost a year ago.

On 3rd December 2015, King Charles and Queen Camilla hosted a dinner in honour of German President Frank-Walter Steinmeier. It was suitably festive: the table was laid in St George's Hall, which was decorated with a 20ft Christmas tree covered in thousands of fairy lights.

The Princess of Wales leaned into the seasonal glitz, opting for sequin lilac creation by Jenny Packham. But the shimmering gown paled in comparison to the tiara that was placed atop Her Royal Highness' head.

The Princess of Wales made her debut in the Queen Victoria Oriental Circlet Tiara, a treasure from the royal collection that was created by Garrard for Queen Victoria in 1853 under the direction of her beloved husband and consort, Prince Albert. The piece was loved by Queen Elizabeth, the Queen Mother, but was only worn once by Queen Elizabeth II and has only been seen a handful of times in recent decades.

'The Oriental Circlet was originally set with a collection of opals, one of Albert’s favourite gemstones, and 2,600 diamonds,’ Garrard explains. ‘Later, believing that opals were unlucky, Queen Alexandra replaced them with rubies. However, the magnificent, Indian-inspired design of lotus flowers and Mughal arches remains largely the same today as when it was first created.’

The stunning tiara was inherited by Victoria and Albert's daughter-in-law, Queen Alexandra. However, she had different ideas about the stones that should take pride of place. Owing to superstitions, she chose to replace the ‘unlucky’ opals with a collection of Burmese rubies given to Queen Victoria as a gift from the ruler of Nepal in the 1870s. Garrard notes, ‘It is also noted in our Royal Ledger that while the opals were being replaced, some of the arches were also removed to make the tiara smaller. Despite this, she is believed to have only worn it once on a state visit to Germany.’

The Oriental Circlet was not seen in public again until it was passed to Queen Elizabeth, The Queen Mother in 1936. ‘Photographed wearing it on numerous occasions, it quickly became one of her favourite jewels,’ notes Garrard. ‘When the celebrated British photographer Sir Cecil Beaton captured portraits of the Queen in 1939 following her husband King George VI’s accession to the throne, in 1950 to mark her 50th birthday and in 1953, just a few weeks before the coronation of her daughter, Queen Elizabeth II, she chose to wear the Oriental Circlet each time.’

‘After the death of King George VI in 1952, the tiara should have passed down to Queen Elizabeth II. However, realising how much her mother loved the piece, she allowed her to keep it. When the Queen Mother passed away in 2002, Queen Elizabeth II inherited the Oriental Circlet. She wore it only once during her reign, on a trip to Malta in 2005.’

The Princess of Wales' decision to wear the tiara to the German state banquet sparked countless headlines and social media posts as royal watchers delighted in laying eyes on this lesser-seen tiara. Could Catherine choose a similarly surprising creation for tonight's state banquet? Watch this space.