Thank you for posting the link to the portrait, Erik. Just in case I'm not the only one who at first sight missed that Ursula posted the link to the b&w engraving in her reply: http://royal-magazin.de/?p=1934
This engraving shows a more complete view of the princess sitting in a chair. Since an engraver wouldn't complete a motive taken from a painting it's safe to assume that there must be a shared original from which both the engraving and the oval painting derive from.
Here's the (mirrored) oval painting overlaying the engraving (I hope, Ursula doesn't mind!).
Medieval revivalism could well be (and I'm inclined to trust your gut feeling more than mine), but whether Roman, Etruscan or Medieval revivalism, it raises the question if the earrings were indeed assembled from spare parts in the mid 20th century or rather originate in the 19th century, either as part of the parure from the beginning or as a later addition.
To me they just don't look like something created in the 1950's to complete an existing parure. But I might underestimate the ability of the goldsmith and designer to get into the spirit of 19th century revival fashion.