I was able to find this photo of Queen Olga of Greece wearing pearl clusters in her headdress and on the bodice of her dress. They appear to be clusters made of pearls and diamonds.

https://www.pinterest.com/pin/410672059756338776/?amp_client_id=CLIENT_ID(_)&mweb_unauth_id=&_url=https%3A%2F%2Fwww.pinterest.com%2Famp%2Fpin%2F802625964814280014%2F

I was also able to find this painting of Queen Olga. She seems to be wearing one of her pearl clusters pinned to her neck along with a rope of pearls as a necklace.

https://www.akpool.de/ansichtskarten/28543862-kuenstler-kabinettkarte-koenigin-olga-von-griechenland-portrait-olga-konstantinowna-romanowa

https://www.greekroyalfamily.gr/picture-gallery/vasileis-georgios-a-a-olga.html

In this portrait of the Queen, which was painted in the early years of her reign, she is depicted wearing a pearl pendant brooch, similar to the one that belongs to Queen Elizabeth II. Looking at the brooch, I can’t help but think that maybe she is wearing the top portion of said brooch in the previous picture?
Another question, would be the subject of the fate of her jewels. We know that Constantine inherited her emeralds; Nicholas, her rubies; George, her pearl brooch, and Christopher, her turquoises. If we go by the pattern made from this distribution, could it be that George also inherited these pearl clusters and brooches? The Queen had two more children, Andrew and Maria, what did they end up with?
Have a nice day y’all!