United Kingdom, April 20 -- 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 througho...