The balance-spring is formed by a first assembly, comprising the coils (3) and the outer curve (2), made in a first material such as Elinvar having an elastic torque that is insensitive or almost insensitive to elongation, temperature and magnetic field variations, and by a second assembly secured to the first assembly, comprising in particular the inner curve (4) made of a second material essentially selected for its mechanical properties and for the shapeability of said inner curve (4) along the most favourable contour for concentric expansion of the balance-spring, such as a Grossmann curve (14), said second assembly also being able to comprise a collet (5).
