Blue topaz is the stone in the anniversary Royal Oak gem set where hardness is the most misleading number. Topaz sits high on the hardness 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 regardless of how resistant it is to scratching. A cutter working topaz has to orient every stone so that the cleavage plane does not run parallel to a table or an edge, and a setter has to apply pressure without ever loading that plane. Doing this across the dial, the bezel and a full bracelet is a different kind of problem from the one tanzanite presents, since tanzanite is simply soft and behaves predictably while topaz is hard right up to the moment it is not. Blue topaz is also the third of the three blue stones in the set, alongside blue sapphire and tanzanite, in the only colour Audemars Piguet represented more than twice.
The case is 41 mm across and 11.6 mm thick in white gold, with the octagonal bezel gem-set in baguette-cut blue topaz. The height runs above the 10.5 mm the selfwinding Royal Oak reaches at the same diameter, and the extra material is setting rather than movement, since each baguette sits in a channel cut into the metal with enough left below to hold it. The eight hexagonal screw positions remain, and the continuous run of stones breaks and resumes at each. The dial is white gold set with baguette-cut blue topaz across its full surface and carries no applied hour-markers. The stones are the dial. Only the white gold Royal Oak hands sit above them. The bracelet carries the same setting to the clasp, tapering from the case and closing on an AP folding clasp. Blue topaz is the palest and cleanest of the three blue stones, lighter than sapphire and without the violet shift of tanzanite, and against white gold it produces the coolest surface in the entire set. The watch reads as ice rather than as colour, which is the opposite of what the warm stones do and is closer to the effect a diamond-set Royal Oak achieves while still being unmistakably coloured.
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 at the point where the run most needs to remain unbroken, and on a cleavable material a cut edge is a liability as well as a visual break. Centre seconds keeps all three hands on one 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 offers no reference points. This is the least favourable legibility pairing in the set alongside pink tourmaline, because pale blue stone and white gold hands sit close in tone and the hands are found by shape rather than by contrast. That is inherent to choosing the lightest stone available.
The Royal Oak is the collection Audemars Piguet is built on, and the anniversary gem set is the most sustained gem-setting programme the collection has carried. Ten stones were used: blue topaz, blue sapphire, tanzanite, ruby, emerald, tsavorite, amethyst, pink tourmaline, yellow chrysoberyl and orange spessartite. Each was issued at 41 mm at this reference and again at 37 mm at 15554, producing twenty references on the same construction across two case sizes. Topaz is one of the few stones in the set that also appears in the multi-stone versions of the same case: one of the two non-anniversary 41 mm references pairs graded blue sapphires with Swiss blue topaz, and the other combines black sapphires with tsavorites and smoky quartz. The 15512 handles the same anniversary brief a third way, using brilliant-cut graded blue sapphires with a diamond-set bezel and retaining the date.
This reference suits a collector who wants a gem-set Royal Oak that stays cool. Among the ten stones it is the one that comes closest to a diamond-set piece in effect while remaining a coloured watch, and for a buyer who finds diamonds too neutral and ruby or spessartite too emphatic, it occupies the space between them almost exactly. The pale tone also makes it the most wearable of the set in the sense that matters socially rather than physically, since it reads as a light watch at distance and resolves into stones only up close. Against that, it demands more of the wearer to read and it carries a structural vulnerability that its position on the hardness scale conceals. It appeals to someone who has considered the sapphire and tanzanite versions and wants the blue without the depth of one or the softness of the other. The 41 mm size is the larger of the two the set offers, with the same stone available at 37 mm. The anniversary designation ties production to the occasion.
