Ruby is the stone in the anniversary Royal Oak gem set that suffers most from being cut small. Red in corundum comes from chromium, and how red the finished stone looks depends on how much material the light passes through, which means the same rough that yields a deep red in a large baguette yields something closer to pink in a small one. A 37 mm case set across the dial, the bezel and the full bracelet needs a great many small stones, and holding a true saturated red across all of them requires starting from rough deep enough that the reduction in path length does not tip the colour. That is a harder selection than the 41 mm version of the same reference demands. It is the single respect in which the smaller watch is the more difficult one to build, and it is specific to this stone, since none of the other nine sits on a colour boundary the way ruby sits on the boundary with pink sapphire.
The case is 37 mm across and 10 mm thick in white gold, with the octagonal bezel gem-set in baguette-cut rubies. The plain 37 mm selfwinding reference of the same generation measures 9.1 mm, so the complete setting adds nine tenths of a millimetre, against one and one tenth on the 41 mm version, because smaller stones sit in shallower channels. The eight hexagonal screw positions remain in place and interrupt the run of stones at fixed intervals, which the setting has to accommodate rather than work around. The dial is white gold set with baguette-cut rubies over 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 setting to the clasp, tapering from the case and closing on an AP folding clasp. Red against white gold is the sharpest colour and metal pairing anywhere in the set, since the two share no tonal relationship at all, and at 37 mm that contrast operates over a denser field of smaller stones, which reads as texture rather than as blocks.
The function set is hours, minutes and centre seconds. The date is absent, and the reason is the dial rather than the movement: an aperture cut through a continuous stone surface breaks the setting at the point where an unbroken run matters most, and on the smaller dial the proportional cost of that break would be higher. Centre seconds holds 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 provides nothing to read against. Ruby retains enough depth at this size for the white gold hands to stay clearly separated from the field, which places this reference among the more legible of the ten at either diameter.
The Royal Oak is the collection Audemars Piguet is built on, and the 37 mm anniversary references step through three levels of setting. The 15550 is plain and measures 9.1 mm, the 15551 adds a diamond-set bezel and holds the same height, and this reference sets dial, bezel and bracelet together and reaches 10 mm. Ten stones were used across the gem-set programme: ruby, blue sapphire, emerald, tsavorite, tanzanite, blue topaz, amethyst, pink tourmaline, yellow chrysoberyl and orange spessartite. Each appears at 37 mm here and at 41 mm at reference 15514, giving twenty references on the same construction in two sizes. Red is the only appearance of its colour among the ten, with no second stone to compare it against, where blue is represented three times and green twice. Two further 41 mm references stand outside the anniversary designation and replace the single stone with graded mixtures of black sapphires, tsavorites and smoky quartz in one case, and graded blue sapphires with Swiss blue topaz in the other. Ruby appears in neither of those, which leaves the single-stone references at 37 mm and 41 mm as the only two ways Audemars Piguet issued the colour in this case.
This reference suits a collector who wants an unambiguous statement in the smaller case. At 37 mm a gem-set Royal Oak stops reading as a decorated sports watch and starts reading as a piece of jewellery built on a watch case, and ruby is the stone that leans hardest into that reading. There is no restrained interpretation of it. Choosing red also means choosing the one colour in the set with no alternative, so the decision is between this stone and a different colour entirely rather than between two versions of the same idea. It appeals to a buyer who finds the diamond-set 37 mm references too neutral and wants the setting to carry colour, and to one who wants hardness alongside it, since ruby shares the top of that ranking with blue sapphire alone. The anniversary designation fixes production to the occasion.
