The glittering ring features a 12-carat Ceylon sapphire from Sri Lanka in a royal blue colour.
In its place, Catherine usually wears her eternity ring stack, but this was also absent from Thursday’s appearance.
Various jewellery experts have suggested the change could be due to the fact that her engagement ring is not always practical for everyday wear.
Back in 2020, when Catherine appeared without her engagement ring during a visit to Evelina Children's Hospital, Kensington Palace told Hello! Magazine that the princess had taken off her jewellery to avoid any potential hygiene hazards.
On Thursday, the future Queen left a positive impression after introducing her husband in a non-royal manner.
While at the hospital’s physical activity room on the rehabilitation ward, Catherine, who is celebrating her 44th birthday today, greeted the patient and shook hands before introducing her to her husband, simply saying: “This is William.”
The heir then joked: “We've come to interrupt your gym session.”
Catering told the patient: “It must be nice doing things like this, a good distraction from all of the medical things... it must be nice having the staff here alongside you, helping, supporting and encouraging you.”
