Émail en résille sur verre (French) literally, enamel in network on glass. Enamelling executed by carving on a plaque of glass (usually dark blue or blue-green) or crystal, backed with foil, a design in slightly concave depressions and then lining the depressions with gold leaf and filling them with powdered transparent coloured enamels so as to make a decprative design.
The enamels were fused at a temperature lower than would result in melting the glass. The technique was used for a brief period by a few French enamellers in the first half of the 17th century.
Some pendants were made with plaques so enamelled from designs made, c. 1619-24, by Valentin Sezenius.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson