This image from 1900 shows Katherine Alexandra Duer Mackay (US1797spouse1) a close-up view of the chain of diamonds and emeralds necklace draped across the bodice of her dress.
The caption accompanying this image reads:
Katherine Mackay's chain of diamonds was a new fashion.
This image is a crop from another image of Katherine holding and wearing orchids. Another crop from that same image shows a larger view of this diamond and emerald necklace draped across the bodice of her dress. A third crop from that image shows her wearing a diamond dog collar around her neck and a diamond and turquoise tiara on top of her pompadour.
See Harbor Hill, Clarence Hungerford Mackay's Mansion.