In the photo below the Duchess is seen wearing a very interesting diamond bracelet.
A detail showing the bracelet.
Now for some speculation.
One of the newspaper reports which I saw (but cannot find this morning) described the Duke's diamond tiara as having a diamond cluster centre which could be replaced by a ruby bow.
As this bracelet shows a diamond cluster motif, I wonder if it could be the tiara? The diamonds are reasonably large and so would be impressive on the style of hair do the Duchess favoured at the time.