Balance for timepieces and method for manufacturing the same

A resonator comprising:

• a balance comprising a rim, a hub, and at least one arm that extends continuously in a radial direction between the hub and said rim, a first end of the arm being directly connected to the hub and a second end of the arm being directly connected to the rim, the second end of the arm including a first branch and a second branch each directly connected to the rim, the first branch and the second branch being separate from one another to form a housing between the first branch and the second branch, and a flexible inertia adjustment element is positioned in the housing, the flexible inertial adjustment element extending from the first branch to the second branch, at least one portion of the balance being made of a partially or completely amorphous metal alloy,
• wherein said at least partially amorphous metal alloy is based on an element chosen from the group consisting of platinum, zirconium and titanium, and has a coefficient of thermal expansion comprised between 7 ppm/° C. and 12 ppm/° C. and a monocrystalline quartz balance spring.