Are crown jewels legally state property, then, or do they belong personally to the reigning monarch?

If the Russian crown jewels mentioned belonged to the state, then Empress Maria Feodorovna should have returned them as soon as her husband died and her son succeeded to the throne. As it was, when Czar Nicholas II asked her to submit them, she bristled and refused.

She may have been justified, however, if they were simply heirlooms.

I believe that some queens have bequeathed family jewels to sons or grandsons who were heirs apparent to thrones -- expecting them to give them to their brides later on, when they married. Of course, the understanding was that the said brides were of royal birth, and hence the marriages were dynastically approved.

Queen Paola of the Belgians, for instance, owns a diamond tiara which her husband's grandmother (Queen Elisabeth) gave to his mother (Queen Astrid) when he (the future King Albert II) was born. She loaned it to her own daughter-in-law Mathilde, to wear on her wedding day in 1999.

I'm not sure when exactly the dowager queen gave it to her grandson. But Queen Alexandra of Great Britain gave her grandson, the then-Prince of Wales (the future King Edward VIII) some valuable jewelry she expected him to give to his queen. She did not, however, anticipate him abdicating the throne to marry a twice-divorced American commoner ... I don't believe that Wallis Warfield Simpson came into possession of them.