Ooh, lookie here! Banana Republic has gone and designed a marvelous Crystal Collar necklace, made in the likeness of the $35 Zara piece the Duchess of Cambridge wore to the Royal Film Premiere of Mandela: Long Walk to Freedom.
Update 5/30/21: The Banana Republic necklace is sold out, but I found a similar piece on Etsy by seller masfemme (with 5-star reviews).
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Catherine paired the costume necklace with a cream Roland Mouret gown, the same one she wore to a private event at Claridge’s in May 2012.