The style of setting a gemstone (usually a transparent faceted stone) in a finger ring or other article of jewelry so that the facets of the pavilion are exposed to light, as opposed to a closesetting. There are several variations of such settings that were introduced in the late 18th century and following years, usually having the stone supported by a circle of prongs projected upwardfrom the shank or by an encircling open-bottom collet or by a box setting from which the bottom has been cut away. Also called an 'à jour setting'.
Open Setting

Open Setting
The style of setting a gemstone (usually a transparent faceted stone) in a finger ring or other article of jewelry so that the facets of the pavilion are exposed to light, as opposed to a closesetting. There are several variations of such settings that were introduced in the late 18th century and following years, usually having the stone supported by a circle of prongs projected upwardfrom the shank or by an encircling open-bottom collet or by a box setting from which the bottom has been cut away. Also called an 'à jour setting'.
References
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson



