This is the only configuration in the range where the dial and the case are the same temperature. Nacre takes its color from whatever surrounds it, and on an Everose or yellow gold watch it picks up warmth from the metal and shifts toward it, but a case built from steel and white gold has no warmth anywhere in it, so the nacre has nothing to react against and shows only what the material itself produces. Reference 278344RBR is the 31mm Datejust in a combination of stainless steel and white gold with a domed bezel set with diamonds, and this configuration pairs it with a white mother-of-pearl dial carrying gem-set markers in white gold settings, on the Oyster bracelet.
Nacre is neither a stone nor a lacquer and behaves like neither. It is the lining a mollusc deposits inside its shell, built up in microscopic layers over years, and the color travelling across a finished dial comes from those layers refracting light rather than from any pigment at all. Cut thin and mounted over a solid base plate, the disc keeps the orientation it had in the shell, so every example carries a different pattern and no two of these watches are identical, and how thickly the nacre is laid over that base governs how much depth the dial appears to have. Rolex grades the discs before fitting and rejects a high proportion, since a band falling in the wrong place can leave a marker or the date window sitting where the material shows no depth. In a cool case the pale greens and pinks that surface as the wrist turns are the material's own colors rather than reflections of the metal around it, which is a considerably purer reading of nacre than the warm-metal references give. The markers sit in raised white gold settings whose height keeps them legible against a ground that is never a consistent brightness. The bracelet is the Oyster, three broad flat links brushed across the top with polished flanks, and the date sits at three under the Cyclops lens with the aperture cut directly into the nacre. White gold is also considerably denser than steel, which means the bezel on this reference is noticeably heavier than a steel one of the same dimensions, and on a 31mm case that weight sits at the outer edge where it is most felt. It is the one way the second metal announces itself without being seen.
Cutting a window into nacre without fracturing the disc is a genuine constraint and part of why the material appears on some references and not others. The Cyclops lens above the aperture has been part of the Datejust since 1953, and on a low-contrast ground like this one it does more work than elsewhere, since the printed date sits against a background whose brightness changes as the wrist moves. The date changes instantaneously at midnight rather than creeping through the surrounding hour, the feature the Datejust introduced in 1945 and the one it takes its name from. A single aperture is also part of why the marker ring holds together, since a second would interrupt a pattern depending on continuity. Correction runs through the winding crown without moving the hands, hand winding uses the same crown with no separate position to learn, and nothing the watch asks for requires a tool. The movement is self-winding with a power reserve of approximately 55 hours, an engineering result on a movement built to fit a 31mm case rather than a figure carried down the range, since a smaller barrel has less room to store energy and a proportionally smaller rotor gathers less of it. The crown screws down against the case, and the Oyster closes with a concealed clasp folded away inside the links. Winding by hand runs through the same crown, and a movement sized for this case gathers less from the same wrist movement than a larger one, so the watch does better worn daily than kept aside between outings.
The Datejust arrived in 1945 as the first self-winding wristwatch to show the date in a window, and the 31mm case sits between the 28mm Lady-Datejust and the 36mm original, which makes it the size for buyers who find one too small and the other too large. The 278000 generation replaced the 178000 series in 2018 with a revised case and bracelet. Steel and white gold is the quietest two-metal combination Rolex offers, since white gold sits close enough to steel in color that most people never register the watch as two-tone at all, and a diamond-set bezel covers most of the only surface where the second metal would be identifiable, so the bezel reads simply as diamonds rather than as gold with stones on it. Nacre came to the model much later than the metal combination and has stayed, largely because it does something no lacquer or cut stone can. Within the generation, 278344RBR is the steel and white gold reference with a diamond-set domed bezel. Rolex casts its own gold in a foundry on its premises, which is unusual in the industry and is the reason the brand can hold an alloy to a consistent color across decades of production rather than across a single batch.
The collector this suits wants nacre rather than the effect of nacre. On a warm case the material is partly reporting the metal around it, and here it is reporting only itself, which is a cleaner proposition and one that surprisingly few buyers think to look for. It is also the configuration where no two examples are alike, since the pattern differs on every disc, and it photographs least accurately of anything on the reference because a still image fixes one state of a surface that does not hold still. On the Oyster the bracelet stays plain and unvarying, which leaves the dial as the only thing on the watch that changes at all. Buyers comparing it to the Everose nacre references usually find this the cooler and more literal of the two readings, and choose on that basis rather than on the metal itself. Shell is one of five set grounds on this reference.
