Blue is the only colour Audemars Piguet represented three times in the ten-stone anniversary Royal Oak set, and blue sapphire is the hardest of the three by a wide margin. The set also carries tanzanite and blue topaz, and against those this reference is the durable one: sapphire is corundum, second only to diamond among natural materials for hardness, where tanzanite is soft enough to scratch against ordinary contact and topaz cleaves cleanly along one plane no matter how hard it is. On a watch where the stones cover the dial, the bezel and the entire bracelet, that difference is not academic. It is the reason this reference can be worn in a way the tanzanite version realistically cannot. Sapphire is also the same mineral species as the ruby in the set, separated only by which trace element coloured it, which makes those two references chemically identical objects in different colours.
The case is 41 mm across and 11.6 mm thick in white gold, with the octagonal bezel gem-set in baguette-cut blue sapphires. That height is greater than the 10.5 mm the selfwinding Royal Oak normally runs to at this diameter, and the difference comes from the setting rather than from the movement, since baguettes have to sit in channels cut into the metal with enough material left beneath them to hold. The eight hexagonal screw positions that define the Royal Oak survive the setting, which is the constraint the whole design works around: the bezel has to accept a continuous run of rectangular stones while leaving those eight points intact. The dial is white gold set with baguette-cut blue sapphires across its full surface, and it carries no applied hour-markers at all. The stones are the dial. Only the white gold Royal Oak hands sit above them, and they are the sole element that is not a stone or its setting. The bracelet continues the same treatment along its length, tapering from the case in gem-set links and closing on an AP folding clasp. Because baguette cuts have flat tables and long straight edges rather than the many small facets of a brilliant, the surface returns light in broad planes rather than in scattered points, and the watch reads as a solid field of colour that shifts in blocks as the wrist moves.
The function set is hours, minutes and centre seconds. There is no date, and its absence is structural rather than an omission: a date aperture would require cutting a window through a dial that is a continuous stone surface, breaking the setting at exactly the point where the eye expects the pattern to be unbroken. Removing it lets the dial run to the edges without interruption. Centre seconds keeps all three hands on one axis, which is the least intrusive arrangement available, since a small seconds subdial would have imposed a second circular break on the same surface. Reading the time on this reference depends entirely on hand position against the case, because the dial gives no reference points. Sapphire is dark enough at saturation that the white gold hands stay legible against it, which is not true of every stone in the set. On the pale stones the hands can vanish into the field at certain angles; on blue sapphire they hold.
The Royal Oak is the collection Audemars Piguet is built on, and the anniversary gem set is the most concentrated statement of gem-setting the collection has carried. Ten stones were used: blue sapphire, ruby, emerald, tsavorite, tanzanite, blue topaz, amethyst, pink tourmaline, yellow chrysoberyl and orange spessartite. Each was issued at 41 mm at this reference and again at 37 mm at 15554, so the full anniversary gem set runs to twenty references built on the same construction in two sizes. Two further 41 mm references sit outside the anniversary designation and abandon the single-stone approach for graded combinations, one pairing black sapphires with tsavorites and smoky quartz, the other combining graded blue sapphires with Swiss blue topaz. The 15512 takes a third approach in the same anniversary run, using brilliant-cut graded blue sapphires on the dial with a diamond-set bezel and retaining the date.
This reference suits a collector who wants the gem-set Royal Oak in the version that can actually be worn. Among the three blue stones in the set it is the one that will not be damaged by ordinary contact, and among all ten it shares the top of that ranking only with the ruby. That practicality is worth stating plainly, because a watch set in stones across the dial, bezel and bracelet is otherwise an object that has to be handled carefully, and the choice of species determines how carefully. It appeals to someone who wants saturated colour rather than the neutral reading a diamond-set Royal Oak gives, and who wants it in the most familiar of the coloured stones rather than in one that requires explanation. The 41 mm size places it as the larger of the two options, with the 37 mm reference available for anyone wanting the same stone in a smaller case. The anniversary designation ties production to the occasion rather than to an open catalogue run.
