The Princess of Wales selected the Ceylon sapphire stone set in white gold that is now iconic.
But - the particular ring that Diana chose was not bespoke, meaning that any old commoner with enough money could buy exactly the same ring. This was considered a big royal no-no.
However, clearly Diana was unfazed and continued to wear the ring long after her divorce from Charles in 1996.
Fast forward to 2010, Diana's son Prince William proposed to Kate Middleton with the iconic ring.
Kate was also thrilled to be wearing the controversial sapphire.
'It’s beautiful – I hope I look after it! It’s very special', she said at the time.