According to the UK media, Empress Eugenie's black pearl necklace was worn by Mary, Countess of Ilchester when her portrait was painted by Edward Robert Hughes.
Below is a photo of the painting.
Closer view of necklace
I found the image first and wondered if the painting showed the pearls. Thankfully, I later found a news item stating it did.
Mary, Countess of Ilchester was reported to have worn them when she attended the first State Opening of Parliament for the reign of Edward VII.