Tanzanite shows different colours along different crystal axes, and that single property governs everything about how this reference had to be made. A tanzanite crystal read one way is blue, read another is violet, and read a third is a brownish red, so every baguette in a run must be oriented identically during cutting or the finished surface will drift in colour across its own length. On a 37 mm Royal Oak set across the dial, the bezel and the full bracelet, the stones are smaller and more numerous than on the 41 mm version, which means more individual orientation decisions and more opportunities for one of them to be visibly wrong. Smaller stones also shorten the path light takes through the material, which on tanzanite pushes the reading toward the paler violet axis rather than the deep blue one. Tanzanite comes from a single deposit in the Merelani Hills of northern Tanzania and was unknown before 1967, making it the only stone in the ten with no history in jewellery before living memory.
The case is 37 mm across and 10 mm thick in white gold, with the octagonal bezel gem-set in baguette-cut tanzanites. The plain 37 mm selfwinding Royal Oak of the same generation runs to 9.1 mm, so the complete setting adds nine tenths of a millimetre, against one and one tenth on the 41 mm reference, since smaller stones sit in shallower channels. The eight hexagonal screw positions remain and break the continuous run of stones at fixed points the setting has to work with. The dial is white gold set with baguette-cut tanzanites across its whole surface and carries no applied hour-markers. The stones are the dial. The white gold Royal Oak hands are the only element above them. The bracelet continues the same setting to the clasp, tapering from the case and closing on an AP folding clasp. Because the colour depends on viewing angle, this watch behaves differently from every other stone in the set as the wrist turns: a sapphire dial changes brightness, a tanzanite dial changes colour, and across a bracelet that curves around the wrist the stones are never all presenting the same axis at once.
The function set is hours, minutes and centre seconds. There is no date, and its absence follows from the dial construction rather than from the movement. An aperture would break a continuous stone surface at the point where an unbroken run matters most, and on the smaller dial the proportional cost would be higher. Centre seconds holds 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. At 37 mm the tanzanite runs lighter than it does at 41 mm for the reasons the smaller stones impose, and the white gold hands consequently work harder against it, which is a genuine characteristic of this size in this stone.
The Royal Oak is the collection Audemars Piguet is built on, and the 37 mm anniversary references step through three degrees of 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 together and reaches 10 mm. Ten stones were used: tanzanite, blue sapphire, blue topaz, 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, producing twenty references on one construction in 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, the most recently discovered and the only one that changes colour rather than brightness. Each of the three was also issued at 41 mm at reference 15514, so a collector choosing blue has six references to weigh across two sizes before considering any other colour in the set.
This reference suits a collector interested in the material before the watch. Tanzanite cut in matched baguettes across an entire Royal Oak is a cutting and orientation exercise before it is a design, and the smaller case multiplies that work rather than reducing it. The softness has to be accepted rather than managed: this is the least robust of the ten stones and a bracelet set in it is not a piece to wear without thought. What it offers in exchange is behaviour no other stone in the set can produce, a surface that is a different colour in the afternoon than it was in the morning. It appeals to someone who already owns a harder gem-set watch and wants a second that does something optical, and to someone who prefers a stone with a short, documented and single-source history. The anniversary designation ties production to the occasion.
