I have not noted the first appearance of this golden necklace, but I think it was after her marriage.
This necklace is actually part of a set of golden jewels which also includes a pair of earrings and a matching bracelet.
I do not know the origin of this set (private gift? official gift? personal acquisition?).
Caption: Camilla, Duchess of Cornwall talks with ballet students before attending a Royal Gala performance at the Royal Ballet School on May 13, 2009 in London, England (Photo by Stefan Wermuth/WPA Pool/Getty Images).
Caption: Camilla, Duchess of Cornwall leaves the official residence for dinner on the fifth day of a tour of the Middle East on March 15, 2013 in Riyahd, Saudi Arabia. The Royal couple are on the third leg of a tour of the Middle East taking in Qatar, Saudia Arabia and Oman (March 14, 2013 - Source: Chris Jackson/Getty Images Europe).