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A crystal cut in the form of a cabochon, shallow or domes, that is carved in intaglio on it flat back with a motif that is realistically painted in minute detail and is surrounded by a transparent ground. The deeper the carving, the more pronounced the three-dimensional trompe l'oeil effect, which is sometimes enhanced by a backing of thin layer of mother-of-pearl. The carved motif, for pieces mounted in a circular gold band as a tie pin, cuff links, button, or studs for men, was usually a racing horse, game bird or dog, and for pieces in a brooch or a locket for women, a floral design or a monogram; some examples depict an insect or a coaching scene.
Sometimes two crystals are mounted back-to-back to form a spherical pendant, preferably with the motifs not identical but complementarily depicting the front and back views of the same subject, e.g. the head of a dog. Some examples consist of two or even three superimposed hollow cabochons, each carved with a different motif, thus increasing the effect of perspective.
The technique was originated by Émile Marius Pradier, of Belgium, c. 1860 (he made the only known-signed example). In England it was developed by Thomas Cook in the early 1860s and carried on by his pupil Thomas Bean and the latter's son Edmund and grandson Edgar (d. 1954).
After the popularity of the pieces in the late Victorian era, the high quality deteriorated by the 1920s, when examples were also being made in France and the United States (some modern pieces depicting motor cars and aeroplanes). The crystals have long been identified with the Hancocks firm. The crystals have sometimes been referred to by the misnomers 'Essex Crystal' or 'Wessex crystal', owing to the erroneous assumption that they were decorated c. 1860 by the enamel portrait painter William Essex (d. 1869).
Imitations have been made of carved and painted glass, and even of a glass cabochon above a printed paper design.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson