This is the palest of the twenty references in the anniversary Royal Oak gem set. Blue topaz is already the lightest of the ten stones Audemars Piguet used, and because colour in a transparent gem depends on the distance light travels through it, the smaller baguettes a 37 mm case requires return a lighter result still than the same material does at 41 mm. Nothing else in the programme reads brighter. Topaz is also the stone where hardness is most misleading. It sits high on the scratch resistance scale, above quartz, garnet and tourmaline and level with beryl at its upper end, but it possesses perfect basal cleavage, a plane running through the crystal along which it will split cleanly under a sharp blow no matter how resistant to scratching it is. Every stone has to be oriented so that plane runs safely, and every stone has to be seated without ever loading it, across a dial, a bezel and a full bracelet.
The case is 37 mm across and 10 mm thick in white gold, with the octagonal bezel gem-set in baguette-cut blue topaz. The plain 37 mm selfwinding Royal Oak of the same generation measures 9.1 mm, so the complete setting costs nine tenths of a millimetre here against one and one tenth at 41 mm, because smaller stones require shallower channels. The eight hexagonal screw positions survive and remain the fixed points around which the continuous run of stones is arranged. The dial is white gold set with baguette-cut blue topaz across its full surface, with no applied hour-markers. The stones are the dial. Only the white gold Royal Oak hands sit above them. The bracelet continues the same setting to the clasp, tapering from the case and closing on an AP folding clasp. Blue topaz is cleaner and lighter than either of the other two blue stones in the set, without the depth of sapphire or the violet shift of tanzanite, and against white gold it produces the coolest surface anywhere in the programme. The watch reads as ice rather than as colour, which brings it closer to a diamond-set Royal Oak in effect than to the saturated references, while remaining unmistakably a coloured stone.
The function set is hours, minutes and centre seconds. There is no date, and the omission is structural: an aperture through a continuous stone surface would break the setting where an unbroken run matters most, and on a material with a cleavage plane a cut edge is a liability as well as a visual interruption. Centre seconds keeps all three hands on one axis and avoids the second circular break a small seconds subdial would impose. Time is read from hand position against the case, since the dial gives no reference points. Alongside pink tourmaline this is the least legible pairing in the set, and at 37 mm it is the least legible of the twenty, because the pale stone and the white gold hands sit closest in tone here. The hands are located by shape rather than by contrast, which is the direct consequence of choosing the lightest stone in the smaller case.
The Royal Oak is the collection Audemars Piguet is built on, and the 37 mm anniversary references form a three-step ladder in setting. The 15550 is plain at 9.1 mm, the 15551 adds a diamond-set bezel over an unset dial at the same height, and this reference sets dial, bezel and bracelet and reaches 10 mm. Ten stones were used: blue topaz, blue sapphire, tanzanite, ruby, emerald, tsavorite, amethyst, pink tourmaline, yellow chrysoberyl and orange spessartite. Each was issued at 37 mm here and at 41 mm at reference 15514, giving twenty references on one construction in two sizes. Blue is the only colour represented three times, and topaz is also one of the few stones used in the graded approach as well as the single-stone one, since one of the two non-anniversary 41 mm references pairs graded blue sapphires with Swiss blue topaz.
This reference suits a collector who wants a gem-set Royal Oak that stays cool and stays small. Of the twenty references in the programme it comes closest to a diamond-set piece in effect while remaining coloured, and for a buyer who finds diamonds too neutral and ruby or spessartite too emphatic it occupies the space between almost exactly. The 37 mm case pushes it further in that direction, since the smaller stones read lighter and the whole watch resolves as a pale bright object rather than as a blue one. Against that it asks more of the wearer to read than any other reference in the set, and it carries a structural vulnerability that its position on the hardness scale conceals. It appeals to someone who has weighed the sapphire and tanzanite versions and wants the blue without the depth of one or the softness of the other. The anniversary designation ties production to the occasion.
