All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
All those ornaments should be added to the inheritance of the mother and it should be divided on all her heirs each according to his legitimate share.
The share of the husband from this inheritance is one-fourth, as regards the remaining heirs, you did not clarify them to us so that we can know the share of each heir; so in this case, it is the Islamic court that should study the matter.
As regards taking back your gift after the death of your mother, then this is not acceptable because she had possessed it when she was alive according to what you mentioned in your the question, and after her death, it becomes an inheritance for all the heirs as we mentioned.
Moreover, her promise to you that the ornaments will be for her grandson (your son) after her death should not be taken into account, as he is not an heir because he is prevented from the inheritance by his father (you) and his uncles. So he has no share in her inheritance and he cannot get anything from what she had left, be it gold or anything else, unless she had made a will to this effect. However, it appears that she did not make a will to her grandson or anyone else and even if she had had the intention to do it, she actually did not do so.