SPRON is a proprietary spring alloy developed by Seiko and the Institute for Materials Research, Tohoku University.
It is a special alloy of Cobalt and Nickel developed for mechanical watch springs. Characteristics include high elasticity, durability, corrosion resistance, and heat resistance. It is comparable to Nivarox, produced by Swatch Group and used by a majority of Swiss watch makers.
Seiko is one of only a few watch makers with its own mainspring production, along with Swatch Group, Rolex, and A. Lange & Söhne.