The magician takes a die and throws it into an opaque cup. The spectator is asked to cover the mouth of the cup and shake or rattle the die inside. They are then asked to call aloud whatever number comes up. A Diamond Die allows the number chosen to be predetermined by the magician to make forcing easy.
The die is not loaded or weighted. A Diamond Die
is a specially numbered die. It could easily be switched out for a normal die with a little sleight of hand if the magician deems it necessary.