Kate Middleton was noticeably without her engagement ring when she visited the Great Ormond Street children's hospital.

The large sapphire ring previously belonged to Prince William's late mother, Princess Diana, so it holds a deep sentimental meaning for the couple. When William gave it to Kate in 2010, he said it was his way of making sure that his mother didn't miss out on his wedding day.

The 12-carat oval blue sapphire is surrounded by 14 solitaire diamonds in an 18K white gold setting.

When Lady Diana and Prince Charles divorced, Diana returned the ring to her ex-husband and it was stored in Kensington Palace. After her death, Charles invited his two sons to choose mementos from her possessions. Prince Harry chose the sapphire ring, while William chose a gold Cartier watch. After William decided to propose to Kate, the brothers exchanged mementos, because Harry thought fondly of Kate and both princes wanted her to have their mother's ring.

So why was she without it?

Appearing at the children's hospital for her visit, Kate wore a stunning red coat over a custom-made peach dress, but one obvious accessory was missing.

Rarely seen without her iconic sapphire, the royal was only wearing her simple gold wedding band to the outing.

Visiting the hospital's Mittal Children's Medical Center at its official opening she was greeting young patients and their family.

In accordance with the hospitals policy on minimal jewelry, Kate left behind her sparkly accessory when she attended the event. Since she would be constantly cleansing her hands as she toured the hospital's ward and treatment areas, it was recommended to her to remove jewelry before entering to avoid spreading germs. Rings, particularly ones with stones and other intricate details are a potential source of bacteria in the hospital.

The Duchess of Cambridge put her ring back on after the morning visit when she was seen with the Wimbledon Junior Tennis Initiative later that day.

This priceless piece of jewelry is estimated to be worth $500,000 even though when Charles originally proposed to Diana with it in 1981, it was worth about $47,000. The ring apparently costs more than Queen Elizabeth's, which is said to be valued at $168,480.

Source: The Diamond Store / People / Harper's Bazaar