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The red secret at the heart of the calibers
They barely twinkle, set like cold stars under the bridge of movement. We can often see them through a sapphire caseback, engraved with a word that has become almost legendary: “17 jewels”. Rubies are not there to seduce, but to survive. In a world of micro-forces and minute friction, these precious stones – most often synthetic – are the invisible guardians of precision, durability and watchmaking style.
Why rubies? The science behind poetry
Reduce friction, preserve energy
A mechanical watch is just a ballet of surfaces in contact. With each rotation of a pinion, with each back and forth of the exhaust, steel against steel would wear out, heat up, consume the energy of the mainspring. Ruby (corundum), with a hardness of 9 on the Mohs scale, offers an extremely smooth and resistant surface. Polished in “olive holes” and combined with a drop of watchmaking oil, the stone reduces the contact surface, stabilizes the lubricating film and reduces friction losses. Result: more amplitude, more consistency, more useful power reserve.
Walking stability and longevity
Beyond efficiency, the rubies stabilize the axis of the mobiles, guide the pivots and limit their wear. Less play means less gait drift over the years. In watchmaking, precision is never a show of brilliance: it is a balance that is preserved. Rubies are this quiet foundation.
Where are they hiding? Stone mapping
They are found wherever pressure is high, speed high or consistency crucial. Some points of reference to visualize their role:
- In the gear train: ruby holes guide the pivots of the wheels (barrel, center, middle, seconds, escapement).
- On the escapement: the two lever pallets are made of ruby; they receive and return the impulse. On the balance wheel, the tiny “pulse stone” (ellipse) is also a ruby.
- On the balance wheel: holes and counter-pivot stones (“cap jewels”) reduce axial friction and stabilize the oil by capillarity.
- In the automatic winding: rubies support the returns and inverted wheels, where the alternating stresses are constant.
The “classic” endowment of a well-made manual caliber: 17 jewels. It covers the gear train, the escapement, and the balance counter-pivots. Beyond that, each complication or system (date, automatic winding, indirect second) adds its stones where it makes mechanical sense.
More rubies = better watch? Not so simple
In the 50s and 60s, the race for “jewels” flirted with the absurd: 30, 40, sometimes 100 rubies displayed on the bridge, without any real benefit. A well-designed watch doesn’t need a forest of stones. It needs rubies in the right places, well sized, well polished, well oiled. A cleverly designed 17 jewels will easily outperform a free 30 jewels. The number reassures, the relevance convinces.
Rubies, sapphires: same family, different story
Watch rubies are not gems taken from a mine: they are synthetic corundums, often colorless (sapphire) or tinted pink/red for tradition. Since the Verneuil method at the beginning of the 20th century, the industry has mastered a stone that is pure, homogeneous, economical and ideal for micrometric machining. The choice is not aesthetic: it is a matter of stability, hardness and consistency of supply.
A mechanics of details: shapes and functions of stones
- “Olive” ruby holes: rounded profile which concentrates contact, retains oil, limits friction.
- Counter-pivot stones (cap jewels): placed above the holes, they control the axial play of the pivot and maintain the oil by capillary action.
- Anchor pallets: made of ruby, they receive the shock of each tooth of the escape wheel, with an angle and finish that dictate the effectiveness of the impulse.
- Ellipse (impulse stone): small but crucial, it transmits energy from the anchor to the pendulum.
Add to this the anti-shock systems (Incabloc, Kif type): a spring lyre protects the cap jewels of the balance wheel in the event of a fall, allowing the pivot to refocus after the impact. Without rubies, these protections would lose their finesse and precision.
Bench anecdotes: what the watchmaker sees, what you feel
When working on a watch, the watchmaker first examines the stones of the gear train and the balance wheel: are they clean, intact, without a stray hair of oil? A cracked ruby hole is rare but audible: amplitude at half mast, ticking going out. A poorly polished or poorly oiled palette results in a nervous, energy-consuming watch that moves capriciously. Conversely, clean rubies, fresh and centered oil, and the whole watch breathes: stable amplitude, silky noise, contained drift.
External signs of finesse
Beyond the number, certain clues betray a real culture of stone: “mirror” chamfers around the holes, rubies flush with no burrs, perfectly aligned pallets, oil in a very round meniscus under the magnifying glass. So many details invisible on the wrist, but which we find, unconsciously, in the sensation of regularity and quality.
Received ideas: we unravel
- “Rubies make the watch precious.” — False, these are inexpensive synthetic stones; it’s the implementation that provides value.
- “The more, the better.” — Not necessarily: mechanical consistency takes precedence.
- “Quartz doesn’t need it.” — Some also use them, on the pivots of the stepper motor, for longevity.
In a word: the elegance of the invisible
Rubies are the hidden elegance of fine watchmaking: a simple, almost archaic solution to an eternal problem — mastering friction to tame time. In their discreet shine, we read a philosophy: it is better to polish a surface than to force a mechanism. And if your next watch delights you with its regularity, thank those little red sparks. They do not seek the light; they create duration.
