Like Mauriz I cannot decide whether Lady Ingreste was wearing only the diamond necklace.
She wore the same necklace or combination at another fancy dress ball in 1912. Unfortunately, the photo does nothing to help clarify the issue.
It does, I think, provide a glimpse of the tiara from which the two brooches included in the second auction were taken.
Brooches -- as auctioned. The press noted that they had been taken from a tiara.