A torque-restoring element for an oscillator for a mechanical timepiece and having an oscillator frequency, said torque restoring element comprising a spiral spring body having a number N turnings with an inner terminal end for engagement with a rotational inertial element via a collet, and an outer terminal for engagement with a stationary cock element, and having a width, a height and a total arc length; wherein the spiral spring body includes a core formed from mono-crystalline silicon wafer oriented along the crystallographic axis ; and wherein the spiral spring body includes at least one peripheral coating of a material having a thermal elastic constant different from that of the core of the spiral spring body so as to maintain the oscillator frequency an oscillator including the torque-restoring element substantially insensitive to variations of ambient temperature.