Nivachron is a balance spring developed by Nivarox, the component manufacturing division of the Swatch Group, and is composed of an alloy based on niobium and titanium — two non-ferromagnetic metals. The distinction that matters is this: it is not a balance wheel, as some sources confuse, but the spring that is paired with the balance wheel. The balance wheel is the oscillating organ; the spring is the hairspring that governs its period.
The problem that Nivachron solves is as old as the spread of domestic and industrial electronics: traditional steel springs magnetize easily, and a magnetized hairspring alters the rate unpredictably — the coils move closer or repel each other, the oscillation period changes, precision collapses. The beryllium alloys used for decades solved the magnetism problem but raised questions of workability and, in industrial settings, toxicity. Nivachron is the modern answer: resistance to magnetic fields comparable to silicon, contained production costs, and stable behavior across temperature variations.
On the bench I've found it in ETA Powermatic 80 calibers, in the Sistem51, in Rado's R766 caliber, in Mido's Calibre 72, in Hamilton's H-50. You find it in both entry-level movements and mid-range references: it is a cross-cutting technical choice, not a price indicator. This is a point many retailers overlook.
The magnetism resistance of Nivachron is declared up to about 1,500 A/m in some calibers, but specifications vary depending on the overall movement design: the spring alone is not enough if the rest of the train is not shielded. It's worth clarifying this because the term is often used as a generic sales argument, almost as if it were absolute protection. It is not: it is a significant improvement over conventional steel, and for the vast majority of daily uses — smartphones, magnetic closures, speakers — it is more than sufficient.
I cannot say with certainty whether Nivarox produces Nivachron for manufacturers outside the Swatch Group: the available technical literature systematically associates it with that perimeter, but I have no documents that exclude external licenses.