Antique plique-a-jour enamel is a rarity because it is so fragile; basically it is melted glass without the structure that backs guilloche or cloisonné enamel. In those pieces, glass is inset into a metal support or backing. The beauty of plique-a-jour is that it is like miniature stained glass.
This pendant shows the effect well. The sterling silver design seems to be of a pine cone, with a ginkgo surround. It is marked but the seller notes that the mark is illegible. This sells for USD$471 on Etsy Shop Ruby and Jules.