Thanks for reminding of the twin necklace set; I've just added it to the list.
In regards to the gift, there is a difference between official and private gifts.
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Official gifts are those received in the course of official duties;
they aren't private property of the member of the Royal Family or indeed the Monarch. Instead,
they are held in property of the royal who received them for life and after his/her death, they
are passed to the Monarch (not personally but in right of the Crown, thus effectively becoming Crown property). However, the Monarch of the time has, under the present guidelines, the freedom to decide whether those gifts should be distributed - e.g. should they become part of the Royal Collection or continue to be held by the late royal's successor(s). Assuming the jewels are held by the deceased royal's successor(s), then upon the successor(s) death, the Monarch of the time will again decide what will happen to those jewels.
In case of Princess Anne, it is possible the Queen (or whoever is the Monarch at the time) will allow some of her non-private jewellery to be used by Zara and Autumn for life, but they'll still revert to the Crown upon their death and it is highly unlikely Anne's grandchildren or great-grandchildren will be permitted to have use of them.
In case of Camilla, her children are not members of the royal family and are extremely unlikely to be allowed use of any of Camilla's official jewellery (including the Saudi parures). The only exception is perhaps during Charles' reign, when they'll be the children of the Queen Consort.
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Personal gifts are those given to royals by those they know privately and not on course or in connection with an official duties. T
hose gifts are private property of the royal, unless otherwise stipulated. They can include wedding, engagement, birth, Christmas presents, as well as those received on other notable occasions. Gifts received from friends and acquaintances and valued at less than £150 can also be considered private.
For instance, most of jewellery given to Camilla by Prince Charles (including the gorgeous chokers) can be considered her private property and she can pretty much do whatever she wishes with them (unless they already constitute part of Crown Jewels).
One point though: while
official gifts are not liable to taxes, private ones can be. Similarly, while there will be no inheritance tax on official gifts (because they are, essentially, Sovereign to Sovereign transactions which are exempt from taxes), there can and will be inheritance tax on private gifts which will be part of the estate of the deceased royal.