That depends on who was gifted them. The first lady of the USA than she would not be able to wear them as they would now be at the disposal of the current first lady Melania Trump.
If the jewels were gifted to mrs Obama she still has them and can wear them.
To the best of my knowledge there are limits to the worth of what can be accepted when in office so i don't think any grand jewels were ever gifted.

I think i once read something about the idea of creating a fund or trust to have jewels for the FLOTUS. Somehow most First Ladies prefer to loan bigger jewels for some events and rarely wear big jewels.