By Gemma Strong When it comes to engagement rings, none is more iconic than the one worn by Kate Middleton. The ring – a 12-carat sapphire surrounded by 14 solitaire diamonds and set in 18-carat white gold – previously belonged to Princess Diana. Following her tragic death in August 1997, it was passed down to her sons, Prince William and Prince Harry \- but returned to the public eye some 13 years later. William famously chose to use his mother’s ring when he proposed to Kate in Kenya in 2010 – and since then, it has been a firm fixture on her hand. But owning a historic heirloom such as the …