Generally, grapes are viewed as a symbol of abundance and fertility. In Greek mythology, Dionysus was recognized as a god of fertility and prosperity. Similarly, Bacchus was the Roman god of agriculture, wine and fertility. Both were usually depicted holding or wearing grapes.
In this photo of a gorgeous Victorian shell cameo, there is a carving of Dionysus, the Greek god of wine, identifiable by the grapes and vine leaves on his head.
It is the theme of abundance that is clearly displayed in a lot of antique jewellery. If you look at the detail of this antique Australian bracelet below, you can see a lovely display of foliage with leaves, tendrils and, in the centre, a bunch of grapes.
Of course, grapes may be chosen for a piece of jewellery for no other reason except that they look pretty and they suit the material being used. In the photo below, coral earrings in the shape of a bunch of grapes look pretty but probably have no other significance or message.