The Duchess of Cambridge (Kate Middleton) owns 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 Net-A-Porter.com:
The ‘Ballon Bleu de Cartier’ watch gets its name for two reasons – the spherical case is reminiscent of an inflated balloon and the cabochon-topped crown appears to be floating. Making its debut back in 2006, this version is crafted from durable stainless steel and has an invisible clasp-fastening bracelet. It’s detailed with black Roman numerals marking the hours on the silvered guilloché opaline dial and signature blued steel sword-shaped hands. It’s a true heirloom that you can pass down through generations.