Of what material are the control rods of a nuclear reactor made?

 The control rods of a nuclear reactor are made of chemical elements, such as boron, silver, indium and cadmium.

Explanation

The control rods are used in nuclear reactors by inserting them into the core and adjusted in order to control the fission rate of the nuclear chain reaction of uranium or plutonium and, thereby, the thermal power output of the reactor, the rate of steam production, and the electrical power output of the power station. 

These rods are made of chemical elements, such as boron, cadmium, silver, or indium, that are capable of absorbing many neutrons without fissioning themselves. All these elements have different neutron capture cross sections for neutrons of various energies.