The Duchess of Cambridge (Kate Middleton) owns and wears the silver coloured stainless steel Cartier Ballon Bleu watch. She wears the 33mm size.
About the Cartier watch:
The watch mechanism is set in a stainless steel case and attached to a stainless steel bracelet. The watch has a silvered opaline dial with blue sword-shaped hands and Roman numeral hour markers and a blue synthetic spinel cabochon.
Here’s how the watch is described at Saks Fifth Avenue:
From the Ballon Bleu de Cartier Collection. Distinctive stainless steel design with blue hands and large Roman numeral dial.