That embargo was just a product of my imagination. I tried to find out why suddenly 30 years after the death of Queen Frederika the fabulous diadem appeared on Crown Princess Pavlos.
Somehow in my mind I got the idea that maybe Queen Frederika has left the diadem to her grandson Crown Prince Pavlos under the condition that the wearer must be of royal or noble descent. That condition was secured with an embargo on the use of the diadem. After 25 years, or thirty years, the embargo ended and the Crown Prince's non-royal or -noble spouse could wear it.
But that was just purely my imagination. I have stated that any other guess is as good as it is. If there is an embargo on the use of royal jewels in Belgium, who has made that condition and why?
Queen Frederika was very much into matchmaking and wanted her children to marry equally (Sophia with Juan de Borbón, Constantine with Anne-Marie of Denmark, Irene was dating Michel d'Orléans). But such an embargo was pretty useless: if her grandson wanted to marry a commoner, I don't think a ban on a diadem would prevent him doing so.
What would be the reason for Princess Lilian to bequeath her eventual jewels under an embargo (a time slot). For an example, these can not be worn as long as Queen Fabiola is still alive?