The Chaumet Choker Tiara

Chaumet are very old French jewellers (they were founded in 1780) and they made this tiara for the Grand Duchy of Luxembourg in the 1980's.

It's actually a trellis choker necklace that can be turned into a tiara by adding pearl buttons to it. Here is a close-up of it:

source

Here is Grand Duchess Maria Theresa wearing it as a choker:

source

And here is Princess Stephanie wearing it as a tiara:

source