Ruby

In watchmaking, ruby serves two distinct functions that must not be confused: technical and decorative. Mixing them is a mistake you pay for at the counter when evaluating a piece.

The technical function is that of bearings, called jewels in English. These are synthetic rubies — aluminum oxide with traces of chromium, grown in the laboratory according to the Verneuil process since the late nineteenth century. Synthesis guarantees uniform hardness (9 on the Mohs scale), absence of porosity, and reproducible geometries with micrometric precision. Used as seats for the balance staff, wheel pivots, and escapement, it measurably reduces friction and slows wear. A conventional wristwatch movement mounts between 15 and 25; complex calibers require more. The 1942 Elgin catalog already indicated 15 jewels as an excellent standard for a man's watch. The number matters, but where they are positioned matters more: a ruby on a balance staff pivot is worth more than one on a slow arbor. The conical jewel of the Greubel Forsey Hand Made 2 is an extreme case — a hand-cut conical synthetic ruby, paired with a roller to measure the residual energy of the spring — and demonstrates how far the specialization of this material can be pushed. The mini ruby rollers of the Bovet Récital 26 Brainstorm, adjustable with a micrometric screw on a radial guide, work on the same principle: minimizing friction without reducing power reserve.

The decorative function, on the other hand, concerns natural ruby, corundum with chromium in varying concentrations, which produces hues ranging from cherry red to bright pink. No two specimens are identical. Cartier has built a precise visual code around this material: the ruby cabochon on the crown — and sometimes on the caseback — identifies models in 950 platinum, while white gold models bear a sapphire. It is not an arbitrary choice: the chromatic contrast between the deep red and the cold white of platinum is intentional. The beaded crown with ruby cabochon of the Tank Cintrée Réédition is the most recognizable example. In different contexts, such as the H. Moser Globetrotter, a ruby structurally fixes the arched bridge on the axis of the map, simultaneously assuming a symbolic value — the North Pole — and a real mechanical function.

The term heart of ruby indicates a selection for maximum red saturation, used on dials and counters where the color must remain legible from a distance. It does not correspond to an official gemological classification: it is a commercial denomination.