The hairspring is the regulating organ of a mechanical watch: it determines the oscillation frequency of the balance wheel and, consequently, the precision of the movement. Without it, the escapement has no time reference. It is an extremely thin component, wound in multiple concentric coils, fixed to the balance wheel via a collet and to the bridge via a cochet. Any intervention on it requires years of practice to be executed correctly.
Material is the first variable. Traditional hardened steel hairsprings, used for decades, are sensitive to magnetic fields: the coils can adhere to each other or to adjacent components, falsifying the oscillation and altering the escapement. For this reason, the industry has developed alternative alloys. Nivachron, developed by Swatch Group and employed in Powermatic 80 and Sistem51 calibers, is a non-ferromagnetic composite alloy that offers superior magnetic resistance without requiring structural modifications to the movement. Rolex responded with Syloxi, a proprietary silicon hairspring used in the 2232 caliber and in the 1908 and Land-Dweller movements: unusual for the brand on compact calibers. Silicon, also adopted by Omega in the latest generation Co-Axial calibers, is non-magnetic, lightweight, and requires no lubrication, but is fragile and expensive to produce. TAG Heuer took a different route with the TH-Carbonspring, created through chemical vapor deposition with carbon nanotubes infiltrated with amorphous carbon: lighter than silicon, homogeneous, non-magnetic, with the collet formed directly during material growth.
The geometry of the hairspring matters as much as the material. The flat hairspring is the basic form: the coils expand on a horizontal plane. The cylindrical hairspring, used by Ferdinand Berthoud as early as 1793 on Louis Berthoud's pocket watch No. 26, expands concentrically in three-dimensional space and ensures more uniform oscillation, but takes up more space and is enormously more difficult to produce. The Breguet hairspring, or overcoil, involves the outermost coil being raised and bent toward the center: it improves the concentricity of expansion and isochronism, namely the hairspring's ability to maintain equal times at different oscillation amplitudes. Having both a Grossmann curve at the inner end and a Phillips curve at the outer end is rare: both require precise manual bending and significantly increase the quality of the oscillator. Moritz Grossmann's 216TMZ caliber and Moser's HMC 805 flying tourbillon both employ both terminal curves.
Those who produce hairsprings internally are still a small minority. Minerva produces them at Villeret in minimal volumes, reserving them exclusively for its own movements. Moser produces them in-house also for the double hairspring tourbillon. Most of the industry depends on a few specialized suppliers. From my workbench, I have seen how a hairspring distorted by even a few hundredths of a millimeter is sufficient to render a movement unusable: it is the component where quality truly shows.