Mother-of-pearl is not a homogeneous material: every disc cut from the shell is different from the next, and this variability is precisely the problem I must manage before even considering aesthetics. It is the innermost layer of certain bivalve mollusks — oysters, murex shells, nautili — composed of aragonite crystallized in microscopic lamellae. These lamellae, arranged in overlapping layers with thicknesses on the order of nanometers, produce the iridescence: light refracts between one layer and another and returns colors that change with the viewing angle. It is not a tint, it is a physical phenomenon.
In watchmaking I work primarily with two types. White mother-of-pearl comes from the common oyster and gives warm reflections, from white to pink to cream. Black mother-of-pearl — more properly gray — comes from Pinctada margaritifera, the black-lipped oyster of French Polynesia: its iridescent spectrum ranges from green to violet to blue, with peaks that no pigment can replicate. I have held discs from both sources in my hands: the Tahitian has a visual density that the white does not.
The technical challenge is fragility. Mother-of-pearl cracks under pressure, chips when cut, does not tolerate sudden thermal variations. A standard dial thickness hovers around 0.3–0.4 mm — some go down to 0.22 mm for layering effects — and at these measurements every machining error is irreversible. Guilloché on mother-of-pearl, which is done, requires very low pressures and tools sharpened with maniacal care; miss the angle by a degree and the surface breaks.
The decorative techniques that exploit mother-of-pearl are varied. Marqueterie assembles fragments cut to form figurative or geometric patterns. Nacre brouillé applies colored varnish to the back of the disc with a horsehair brush, letting the color filter through the translucency of the material and create unreproducible irregular clouds. Painted miniature uses mother-of-pearl as support for overlapping layers of tempera colors, baked in an oven. The Japanese raden technique inlays mother-of-pearl fragments beneath layers of natural lacquer.
An aspect often overlooked: mother-of-pearl is semi-transparent. Citizen exploits this in Eco-Drive dials by letting light pass through the disc to power the solar cell beneath. Other manufacturers paint the back of a white disc with colored lacquer to achieve tones — sky blue, green, blue — that natural mother-of-pearl does not possess. The color is not in the mother-of-pearl: it is beneath it.