The Queen's portrait is updated from time to time and she is shown wearing a necklace on some �2 coins (the last were minted in 1997). However there are millions of them and they are only worth their face value - �2.
This old chestnut is answered in several places on various websites, but the best and most complete answer is at:-
www.24carat.co.uk/twopoundsrumour.html