The Great British Bake Off presenter paired the budget bling with a black cut out dress, while the Duchess of Cambridge teamed hers with a cream Roland Mouret floor-sweeping gown with a thigh-high split.
The necklace promptly sold out and the morning after her appearance, it was unavailable online.
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It's not the first time queen of baking Mary has been spotted in Zara. The chef caused a stir when she wore a floral bomber on the Great British Bake Off.
Viewers were quick to tweet their approval of Mary's multicoloured silk jacket and complemented her on her daring look.
The jacket soon sold out of Zara stores nationwide and shortly after, the £29.99 item was appearing on auction site eBay for up to £200.
Speaking recently about her appearance, the grandmother-of-five said, "Obviously I want to look good. You'll always see me blonde, I'll tell you that much!"