This is reportedly the nickname King Charles III gave Meghan Markle

Royal expert Katie Nicholl, author of the book ‘The New Royals: Queen Elizabeth’s Legacy and the Future of the Crown’ talked about the nickname that King Charles III gave Meghan Markle before she officially married his youngest son, Prince Harry.

Apparently the King decided to refer to the Duchess of Sussex as ‘tungsten‘, a nickname that, according to Nicholl, is due to the hard personality of the former royal, as it is the name of a metal that is characterized by its extreme strength and difficulty to break.

It is certainly a very particular nickname that could easily have given an indication of the feelings of the head of state regarding Meghan Markle, who showed to be a rather closed person with strong character from the beginning of her days in the royal family until today; this prevented her from having a good relationship with the king.
At the moment it is not known if King Charles III still refers to Meghan Markle as tungsten since she abandoned her royal duties to move to the United States with her family, and apparently there is no communication between the monarch and the Duchess of Sussex in current days.