I can see certain design elements of Cartier - but that doesn't necessarily mean it was made by Cartier, but well worth further investigation. Does it have a maker's mark or any marks anywhere? Any paperwork or history of provenance?
The broach does have toned down, semi-likeness to a sapphire, diamond, emerald, amethyst and ruby brooch by Cartier, London.
This royal broach was given by the Duke of York (the future King George VI) to the Duchess of York (the future Queen Elizabeth, later The Queen Mother) in 1928 and later handed down to Princess Elizabeth by her parents as a birthday present during the Second World War. You can see the broach at the following link.
