Famously, this is the tiara Kate Middleton (now Catherine, Princess of Wales) wore on her wedding to Prince William.
This tiara was bought in 1936 by the Duke of York (later George VI) for his wife.
It features 16 graduated scrolls, made of over 700 small diamonds. As tiaras go, it's a smalll demure piece.
It was given to the 18-year old Princess Elizabeth (later Elizabeth II) as a birthday present. Elizabeth II was never photographed wearing it; it was not to her taste (though her sister Princess Margaret wore it occasionally in her twenties, most famously to Elizabeth II's coronation). It was then worn by Princess Anne in the late 1960's/early 70's. It wasn't seen again till Kate Middleton wore it for her wedding in 2011.
Here they all are, wearing it: