It was mentioned by Court Jeweler Bolin in an interview two years ago that the amethyst tiara remains convertible and could be worn as necklace again one day. Queen Silvia confirm this in the TV documentary - she says that the setting of the piece itself wasn't altered, it was merely a tiara frame made to hold it.
--Previous Message-- : we got to see it without its frame so it could : simply be used as a necklace again. :