The "controversial" story behind Princess Diana's engagement ring has been revealed.

Now one of the most recognisable pieces of jewellery in the world, it's been revealed in report by People her band was once considered a major break from royal convention.

Princess Diana, who was 20 when she became engaged to Charles, now King Charles III, 77, selected the ring herself from a catalogue at Garrard, the royal jeweller, rather than commissioning a bespoke design.

The piece - a 12-carat oval Ceylon sapphire surrounded by 14 diamonds - was valued at more than $37,000 at the time and was available for public purchase, a detail that set it apart from traditional royal engagement rings.

Diana wore the ring throughout her marriage and was occasionally seen wearing it after her divorce in 1996.

In later years, the ring became a defining symbol of Diana's style and influence. Sara Prentice, creative director at Garrard, has said about the piece: "All of our designs at Garrard are rooted in our heritage, and we continue to capture the elegant classicism of Princess Diana's engagement ring in our 1735 jewels."

Diana's choice was seen as unconventional because it departed from the royal preference for custom-made jewellery.

But the design itself had historical roots.

It was inspired by a sapphire brooch given to Queen Victoria by her husband in 1840, which was later worn by Queen Elizabeth II.

Victoria is said to have worn the brooch on her wedding day as her "something blue".

Following Diana's death in 1997, the ring passed to her younger son, Prince Harry, now 41, who later gave it to his brother, Prince William, now 43, when William proposed to Catherine, Princess of Wales, 44, in 2010.

William spoke about the decision during an interview with ITV.

He said: "Obviously, (Diana's) not going to be around to share in all the fun and excitement, so this is my way of keeping her close to it all."

Harry later incorporated elements of Diana's jewellery into the engagement ring he gave to his wife, Meghan, Duchess of Sussex, 44.

Meghan said in an interview with the BBC: "It's so important to me to know that she's a part of this with us. It's incredibly special to be able to have this, which sort of links where (Harry comes) from and Botswana, which is important to us. It's perfect."

Sara Prentice later reflected on Diana's influence in a separate interview with The Times.

She said: "Princess Diana was my muse for this collection, not just because of her Garrard cluster engagement ring, but because of her style, her elegance and her attitude. Diana was a trailblazer and Blaze is modern and bold, just as she was."

Diana's ring has since influenced jewellery design and has been replicated in various collections, with sapphire engagement rings becoming more widely adopted by a series of celebrities.