The necklace was made by Georg Jensen, a Danish company that makes possibly the best hand made sterling silver jewelry and flatware in the world. (Lately, they make some gold, too, but they really are about silver.) The grape motif is one of the classic Jensen designs. The necklace is not gold, and it is not jeweled, it is just sterling silver, and Mary is probably wearing it because it is Jensen and she represents Denmark.
A necklace like this one would probably cost about $3,000 today, although the company has been in business since 1904, and the one Mary is wearing may be old. One of the great things about Jensen is that they make things today the way they always made them; they have maintained quality. It would be difficult to tell the difference between an old piece and a new one.
