this picture was precisely taken in 1960, during Charles de Gaulle's State visit to Britain (for the State banquet at Buckingham Palace). Certainly, it was a very special occasion for her to wear such a bling!
We can also notice that Queen Mother Elizabeth wore a foreign order (in this case the red sash of the French Legion of Honour), which she rarely did during her long widowhood - even during later French State visits.
Here are a few more pictures of De Gaulle's State visit - only the gala at Covent Garden, which seems to have been covered by the media more than the other gala events.