A very interesting idea, thank you LauraM for this information.
Has anyone seen the actual portrait in the National Portrait Gallery, UK? A life viewing would help to solve the riddle.
The image on the NPG's website is too small for me to form any definite opinion; however, when I zoomed in on the image Baxter posted in the banner, it would appear that there is a rectangular gem between the two larger ones, and I suspect that the odds of finding two rectangular pearls of similar size would be small.
If the headdress in Baxter's image does not include opals, we still have to find an opal tiara which is not the Oriental circlet and which pre-dates it.
We also now know of a previously unidentified pearl parure.