Tanzanite is the most difficult stone in the anniversary Royal Oak gem set to execute in matched baguettes, and it is difficult for a reason specific to the species. Tanzanite is strongly pleochroic: a single crystal shows blue, violet and a brownish red depending on which axis it is viewed along, which means every stone in a run has to be oriented identically during cutting or the finished setting will show colour drift across the surface. On a watch that carries stones on the dial, the bezel and the entire bracelet, that is a large number of stones all required to agree. Tanzanite is also the softest of the ten stones Audemars Piguet used, soft enough that ordinary contact will mark it, and it comes from a single deposit in the Merelani Hills of northern Tanzania and nowhere else on earth. It was unknown until 1967. Of the ten stones in the set, it is the only one with no history in jewellery before the middle of the last century.
The case is 41 mm across and 11.6 mm thick in white gold, with the octagonal bezel gem-set in baguette-cut tanzanites. The height exceeds the 10.5 mm a standard selfwinding Royal Oak runs to at the same diameter, and the difference is the setting rather than the movement, since baguettes sit in channels cut into the metal that need material left beneath them to hold. The eight hexagonal screw positions are retained, and the continuous run of rectangular stones has to break and resume at each of them. The dial is white gold set with baguette-cut tanzanites across its full surface, with no applied hour-markers of any kind. The stones are the dial, and the white gold Royal Oak hands are the only thing above them. The bracelet continues the setting to the clasp, tapering from the case and closing on an AP folding clasp. Because tanzanite shifts between blue and violet according to viewing angle, this reference behaves differently from every other stone in the set as the wrist moves. Where a sapphire dial changes brightness, a tanzanite dial changes colour, and the effect is a surface that is not the same colour twice in the course of an afternoon.
The function set is hours, minutes and centre seconds. There is no date, and the omission is a consequence of the dial rather than of the movement: an aperture would have to be cut through a continuous stone surface, breaking the setting at the point where an unbroken run matters most. Centre seconds keeps all three hands on one axis and avoids the second circular break a small seconds subdial would impose. The time is read from hand position against the case, since the dial provides no reference points. Tanzanite sits at a middle depth of tone, lighter than sapphire and darker than topaz, and the white gold hands hold against it in most conditions but can flatten into the field when the stones swing toward their paler violet axis. That is a real characteristic of the stone rather than a flaw in the watch, and it is the price of the colour behaviour that makes this reference distinct.
The Royal Oak is the collection Audemars Piguet is built on, and the anniversary gem set is the most concentrated gem-setting exercise the collection has carried. Ten stones were used: tanzanite, blue sapphire, blue topaz, ruby, emerald, tsavorite, amethyst, pink tourmaline, yellow chrysoberyl and orange spessartite. Each was issued at 41 mm at this reference and at 37 mm at 15554, giving twenty references on one construction across two sizes. Blue is the only colour represented three times, by tanzanite, blue sapphire and blue topaz, and of those three tanzanite is the softest and the most recently discovered. Two further 41 mm references stand outside the anniversary designation and replace the single stone with graded combinations, one of black sapphires, tsavorites and smoky quartz, the other of graded blue sapphires with Swiss blue topaz. The 15512 approaches the same anniversary brief differently again, with brilliant-cut graded blue sapphires and a diamond-set bezel, and retains the date.
This reference suits a collector who is interested in the stone as much as in the watch. Tanzanite from a single deposit, unknown before 1967, cut in matched baguettes across an entire Royal Oak is a sourcing and cutting problem before it is a design, and a collector who understands that is buying something different from a collector choosing a colour. The softness has to be accepted rather than worked around: this is not a watch to wear without thought, and the same properties that produce the colour shift make the surface vulnerable. It appeals to someone who already owns a harder gem-set piece and wants one that behaves optically rather than one that simply holds colour, and to someone who values a stone with a short and specific history over one with a long and general one. The 41 mm size is the larger of the two the set offers, with the 37 mm reference carrying the same stone. The anniversary designation ties production to the occasion.
