I can't just locate it, but there was a picture of the Princess Royal at a recent evening function wearing a dark blue, high necked gown with what appeared to be a candidate for the brooch on the bodice. The only other thing I could think would be a candidate as the clasp to the pearl choker from Empress Marie Feordorovna but I don't recall that having a brooch clasp.
--Previous Message-- : : it is Menkes who says - : : Prince Albert made copies of the brooch for : his own daughters, and the Queen has given : one of these to the Princess Royal. : : - but no reference source is supplied for : that assertion. : : :