I believe that the necklace which you posted is the same as the one DOC wore...and I also believe that it wasn't altered...they just removed the 'outer' part of the necklace (the longer one with pearls and amethysts)...if you look at my side closeup, you can see another amethyst under her hair...there is total of 4 on each side...so that part of the necklace is the same length..
Interesting that she already wore those amethyst earrings with this antique looking necklace...I wonder if its Queen Mom's as well...