A saturated stone gets harder to match as it gets smaller, and that is the problem this reference solves. Colour in a transparent gem is a function of how far light travels through the material before it leaves, so the same sapphire rough cut into smaller baguettes returns a lighter blue than it would at larger dimensions. Setting a 37 mm Royal Oak across the dial, the bezel and the whole bracelet therefore requires stones selected at a deeper starting saturation than the 41 mm version of the same watch needs, simply to arrive at the same finished colour. Sapphire tolerates this better than most because corundum reaches depths of blue that few other species can, giving the cutter room to work down. It is also the hardest material in the ten-stone anniversary set alongside ruby, and on a smaller case where the bracelet sits closer to the edges of the wrist, that resistance to marking is worth more, not less.
The case is 37 mm across and 10 mm thick in white gold, with the octagonal bezel gem-set in baguette-cut blue sapphires. The plain 37 mm selfwinding Royal Oak of the same generation runs to 9.1 mm, so the full setting costs nine tenths of a millimetre here against the one and one tenth it costs at 41 mm. Smaller stones need shallower channels, and the proportion holds better on the smaller case as a result. The eight hexagonal screw positions survive the setting and remain the fixed points the continuous run of stones has to break and resume around. The dial is white gold set with baguette-cut blue sapphires across its full surface, with no applied hour-markers of any kind. The stones are the dial. Only the white gold Royal Oak hands sit above them. The bracelet carries the same treatment along its length, tapering from the case and closing on an AP folding clasp. Because baguettes present broad flat tables rather than the scattered facets of a brilliant cut, the surface returns light in planes rather than in points, and at 37 mm those planes are smaller and more numerous, which gives the watch a finer grain than the 41 mm version achieves.
The function set is hours, minutes and centre seconds. There is no date. An aperture would have to be cut through a continuous stone surface, breaking the setting at the point where the run most needs to stay whole, and on a 37 mm dial the proportional damage would be greater than on a 41 mm one because the window would occupy more of the available field. Centre seconds keeps all three hands on a single axis and avoids the second circular interruption a small seconds subdial would impose. Time is read from hand position against the case, since the dial gives no reference points at all. Sapphire holds enough depth even at this size for the white gold hands to separate cleanly from it, which is not true of the pale stones in the set at either diameter.
The Royal Oak is the collection Audemars Piguet is built on, and the 37 mm anniversary references form a clear ladder in how far the setting goes. The 15550 is plain at 9.1 mm, the 15551 adds a diamond-set bezel while leaving the dial alone and stays at 9.1 mm, and this reference sets the dial, the bezel and the bracelet and reaches 10 mm. Ten stones were used across the gem-set anniversary programme: blue sapphire, ruby, emerald, tsavorite, tanzanite, blue topaz, amethyst, pink tourmaline, yellow chrysoberyl and orange spessartite. Each appears here at 37 mm and again at 41 mm at reference 15514, so the complete gem set runs to twenty references on one construction in two sizes. Two further 41 mm references sit outside the anniversary designation and use graded mixtures instead of a single stone, one combining black sapphires with tsavorites and smoky quartz, the other graded blue sapphires with Swiss blue topaz.
This reference suits a collector who wants the gem-set Royal Oak at the size where it reads as jewellery rather than as a sports watch that has been decorated. At 41 mm the octagonal case still dominates and the stones are applied to it; at 37 mm the proportions tip and the setting becomes the subject. Blue sapphire is the choice for someone who wants that without giving up durability, since it is the hardest stone available in the set and the one least likely to be marked by the ordinary contact a bracelet takes. It appeals to a buyer who has looked at the diamond-set 15551 and wants colour instead of neutrality, and to one who wants the most familiar of the coloured stones rather than one that has to be explained. The anniversary designation ties production to the occasion rather than to an open catalogue run.
