Official gifts given to members of the BRF in the course of their "work" are assessed as to whether that member can use it or it goes straight to "The Crown". As I understand it, the final decision lies with the Monarch and of course the advice given to him by the relevant aides. With jewellery, there are the "Crown Jewels" and then, just like Winsor Castle or Buckingham Palace, the Crown Collection which is comprised of such gifts.
For example, the Ruby, Emerald and Sapphire demi-parures given to Camilla on tour would fall under this category and she would have use of them but they would belong to The Crown while her marvellous collection of pearls are her own personal property. On the other hand, personal gifts are handled differently. HM inherited, among other things, the "Greville Bequest" and while she wore few pieces, her mother adored the Bucheron which was next seen on Camilla who, like the QM, seems to prefer it above others., just as the Lover's Knot last seen on Diana is now worn by Catherine who also seems to prefer it above others. Both of these and more are deemed "lifetime loans" and return to the monarch upon the death of the wearer.
Gifts given for a wedding or birthday are, as I understand it, the property of the recipient with the caveat that if it is diplomatically troublesome it will not be used in the immediate future or, as occurred with Meghan's chandelier earrings, she wore them and their provenance was "leaked". Needless to say, we haven't seen them again.