Here on this picture you see the Duchess of Wellington (HRH Princess Antonia of Prussia) with a magnificent collier, on her turn almost overpowering the Duchess of Cornwall standing aside her... However the collier of the Duchess of Wellington became fussed the neckline of the (beautiful gown). There are examples with success, I know, but in general the best result is when colliers are worn how it is meant: on skin.
On
this picture you see how Queen Margrethe made sure that the cut of her gown would leave room for her collier. Queen Silvia also
does the same. The usually so stylish
Princess Caroline of Monaco really blurred her Cartier Tutti Frutti collier with her gown, while
Princess Máxima had more success with wearing how it should be worn.
Maybe the Duchess of Cornwall should only wear the three-strand version of the Greville collier, esthetically more pleasing, I think, less a diamond breastplate look.