Tsavorite is the one stone in the anniversary Royal Oak gem set for which the smaller case is the easier build. The material occurs in a narrow belt through Kenya and northern Tanzania and it very rarely produces large crystals, which is why tsavorite jewellery is generally assembled from small stones and why large single examples are treated as exceptional. A 37 mm Royal Oak set across the dial, the bezel and the bracelet needs many small matched baguettes, and that is precisely the size range in which tsavorite is available. The 41 mm version of the same reference asks for larger stones from a supply that yields fewer of them. Every other stone in the set gets harder to match as it gets smaller, because colour depends on the distance light travels through the material; tsavorite is the exception that runs the other way, because scarcity rather than optics is its binding constraint.
The case is 37 mm across and 10 mm thick in white gold, with the octagonal bezel gem-set in baguette-cut tsavorites. The plain 37 mm selfwinding Royal Oak of the same generation measures 9.1 mm, so the full setting costs nine tenths of a millimetre here, less than the one and one tenth it costs at 41 mm, because smaller stones require shallower channels cut into the metal. The eight hexagonal screw positions survive and remain the fixed interruptions the run of stones is built around. The dial is white gold set with baguette-cut tsavorites 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 carries the same setting to the clasp, tapering from the case and closing on an AP folding clasp. Tsavorite is a highly refractive garnet and is typically free of the internal fracturing that emerald almost always carries, so the dial holds its green in low light where the beryl version goes dark, and at this size the effect is a fine even field rather than a small number of large bright planes.
The function set is hours, minutes and centre seconds. There is no date. Cutting an aperture through a continuous stone surface would break the setting where the run most needs to stay whole, and on a 37 mm dial that break would occupy a larger share of the available field than it does at 41 mm. Centre seconds keeps the three hands on one axis and avoids a second circular interruption. The time is read from hand position against the case, since the dial offers no markers. Tsavorite sits at a middle depth of tone and the white gold hands separate from it reliably, helped rather than hindered by the brightness of the stone, because the light it returns is coloured and the hands are not.
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 carries a diamond-set bezel over an unset dial and holds the same 9.1 mm, and this reference sets dial, bezel and bracelet and reaches 10 mm. Ten stones were used: tsavorite, emerald, ruby, blue sapphire, tanzanite, blue topaz, 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 a single construction in two sizes. Green is one of only two colours represented twice, in tsavorite and emerald, which makes those two the one direct within-colour comparison the set offers. Tsavorite also appears in one of the two non-anniversary 41 mm references, combined with black sapphires and smoky quartz on a graded dial. It is one of only a few stones in the programme that Audemars Piguet used in both the single-stone and the graded approaches, which gives a collector a direct view of how the same material behaves when it carries a dial alone and when it is one voice among three.
This reference suits a collector who wants green and prefers the stone chosen on how it behaves rather than on what it is called. Emerald carries the history and the recognition; tsavorite carries the brightness, the clarity and a supply story that begins in 1967 rather than in antiquity. At 37 mm the argument tilts further toward tsavorite, since the smaller stones it naturally yields suit the case and the density of the setting reads as an even green surface rather than as a mosaic. It appeals to a buyer who has looked at both greens closely enough to have a reason, and to one who wants a gem-set Royal Oak that stays legible and holds its colour indoors. The anniversary designation ties production to the occasion rather than to an open catalogue run.
