Curator of Decorative Arts
The curator (whose details are above) must be credited as the researcher who discovered that the necklace/tiara contains opals.
A possible explanation for why Lady Louise Burrell was wearing the Opal Necklace/Tiara in 2011 is that the Argyll Family Tiara had been lost at Glasgow Airport by the Dowager Duchess in 2006, and was not found until 2012. It's quite likely that Lady Louise borrowed a Tiara from a member of the Devonshire Family, if they still own the piece, though I can't find any familial links with them. Since then she's worn the Argyll Tiara again, so I would doubt it belongs to her now.
Now knowing that it contained Opals, I find it very interesting how often it was worn by Mary, Duchess of Devonshire, given that she usually alternated it with the larger of the two Devonshire Diamond Tiaras! I personally wouldn't have thought Opals to be that wearable, but she wore them all the time.