À jour
from the Antique Jewelry University
À jour (from French: jour = day) is a term used to describe openworked backgrounds, either by sawing or piercing. À jour settings for gemstones became popular around
1800. In enamelling the term plique à jour is used to describe an item with an open background which is filled with transparent enamel. This creates a
myriad of colors when held to the light.