Camilla appeared with Charles at the event in February 2005. She showed off her engagement ring when the pair arrived for a party at Windsor Castle.
Amethyst has been used in the past to bring calmness and peace.
It is often used for medication by practisers.
Interestingly, the colour has many the stone is associated with nobility and the Royal Family.
As Camilla would soon be entering the Royal Family on her marriage to Charles, it was an apt choice.
She wore the piece for important events like Trooping the Colour.
Camilla’s engagement ring is one of the more interesting pieces in her jewellery wardrobe.
Prince Charles gave her a family heirloom, a piece once belonging to his grandmother, the Queen Mother.
It has been thought for a long time to be one the most expensive in the Royal Family.