Originally Posted by Warren
The Duchess of Cornwall's gifts (eg the Saudi parures) stay with the Crown, as do (we assume) the gifts HM receives. As these jewels will in effect be passed from Sovereign to Sovereign the difference as to whether they are "private" or "state" is sort of moot.
Buckingham Palace (and most Royal Houses) tend not to answer questions about their jewellery collections. Suzy Menkes, in the introduction to her book "The Royal Jewels" states that she was "actively discouraged [by] Buckingham Palace" in some of her research.
All of the jewels received by members of the royal family during a public engagement or state visit are considered their private property, to be disposed of as they wish. Jewels received by The Sovereign or a Queen Consort on a state visit usually are accepted on behalf of the Crown, but there is no requirement to do so.
The Saudi ruby, emerald and sapphire parures presented to The Duchess of Cornwall were intended as private gifts, however, once the press starting harping on it, suddenly they became "on behalf of the Crown" due to the questioning of whether she should pay a gift tax. Never mind that other members of the royal family never pay gift taxes.