The BBC warns that someone is nabbing expensive jewelry across Europe with a slight of hand. It is the case of a thief in headscarf:

A jewel thief dressed as a wealthy Muslim woman has stolen rings worth hundreds of thousands of euros from dealers across Europe, police say.

The woman, wearing a headscarf, coat and gold-framed glasses, is suspected of swapping gems for replicas.

In one incident in Paris, the woman allegedly stole a 5.5 carat diamond ring worth 635,000 euros (£574,000).

The article says it takes a while before staff realize the jewelry has been replaced, and that they often do not have insurance. Classic example of weak controls creating the opportunity for crime. Either the rings are unique and the thief is doing research to replace them with carefully crafted replicas, or these expensive rings are typical examples of bland and uninspired mainstream objects that can be swapped with just about anything and not be noticed.