Ruby and blue sapphire are the same mineral. Both are corundum, and the only thing separating the two references in the anniversary Royal Oak gem set is which trace element entered the crystal as it formed: chromium produces red and is called ruby, iron and titanium produce blue and is called sapphire. That makes this reference and its blue counterpart chemically identical watches, and it also makes them jointly the hardest of the ten stones Audemars Piguet used, second only to diamond among natural materials. Red is the only appearance of the colour anywhere in the set. Where blue is represented three times and green twice, ruby stands alone, and there is no second red stone to compare it against or to soften the choice.
The case is 41 mm across and 11.6 mm thick in white gold, with the octagonal bezel gem-set in baguette-cut rubies. The extra height over the 10.5 mm a standard selfwinding Royal Oak runs to at this diameter comes from the setting rather than the movement, since each baguette needs a channel cut into the metal with enough material left below it to grip the stone. The eight hexagonal screw positions remain, and they are the fixed points the setting has to work around, since a continuous run of rectangular stones across an octagonal bezel has to break and resume at each one. The dial is white gold set with baguette-cut rubies over its whole surface and carries no applied hour-markers. The stones are the dial. The white gold Royal Oak hands are the only element above them. The bracelet carries the same setting along its full length, tapering from the case and closing on an AP folding clasp. Red against white gold is the highest-contrast pairing in the entire set, because there is no tonal relationship between the two at all, and the effect is that the stones read as separate from the metal rather than as continuous with it. Baguette cuts return light across broad flat tables rather than scattering it through many small facets, so the dial behaves as a field of solid colour that changes in blocks as the wrist moves.
The function set is hours, minutes and centre seconds. The absence of a date follows from the dial construction rather than from any decision about the complication: cutting an aperture through a continuous stone surface would break the setting at the one point where the pattern most needs to be unbroken, and Audemars Piguet chose to keep the surface intact. Centre seconds holds all three hands on a single axis, avoiding the second circular interruption a small seconds subdial would have required. Reading the time depends entirely on hand position against the case, since the dial offers no markers to read against. Ruby at full saturation is dark enough that the white gold hands separate cleanly from it in most light, which is a genuine advantage over the paler stones in the set, where the hands can disappear into the field at certain angles.
The Royal Oak is the collection Audemars Piguet is built on, and the anniversary gem set is the most sustained piece of gem-setting the collection has carried. Ten stones were used: ruby, blue sapphire, emerald, tsavorite, tanzanite, blue topaz, amethyst, pink tourmaline, yellow chrysoberyl and orange spessartite. Each appears at 41 mm at this reference and again at 37 mm at 15554, making twenty references built on the same construction across two case sizes. Two further 41 mm references sit outside the anniversary run and drop the single-stone approach for graded mixtures, one combining black sapphires with tsavorites and smoky quartz, the other pairing graded blue sapphires with Swiss blue topaz. Within the same anniversary programme the 15512 takes a different route again, using brilliant-cut graded blue sapphires on the dial with a diamond-set bezel and keeping the date.
This reference suits a collector who wants the gem-set Royal Oak to be unambiguous. Red is the most declarative colour in the set and the only one without a companion, so choosing it is not a choice within a family of similar options but a choice of the single stone that behaves that way. The hardness matters as much as the colour: with stones across the dial, the bezel and the full bracelet, the species determines how the watch can be handled, and ruby shares the top of that ranking only with the blue sapphire version. It appeals to someone who has already decided against diamonds and wants colour rather than brilliance, and who wants that colour stated without qualification. The 41 mm size is the larger of the two the set offers, with the 37 mm reference carrying the same stone for anyone wanting less presence. The anniversary designation fixes production to the occasion rather than to an open catalogue run.
