The material of the balls being used in the Newton's cradle can vary, but the most ideal material is very elastic. A Newton's cradle will last longer with balls made out of elastic material. The better the material of the ball that you are using is bouncy, the higher the elasticity of it is. Elasticity is the ability for material to deform and return to its original shape without losing any energy. Very elastic materials such as the ones recommended for this system lose little energy, while inelastic material lose more.
Stainless steel is both a common and recommended material used for a Newton's cradle due to it being a highly elastic material.