I've been told that the coefficient of restitution is a little like friction; it's a measure of how elastic a collision is between two objects, it's not an individual property of each object itself. Now let's say you have two balls bouncing on the ground. Ball 1 has a coefficient of restitution with the ground of 0.8, and ball 2 has a coefficient of 0.7 between itself and the ground. This way you have the ground as a common reference. Is it then acceptable to assign a coefficient of restitution to each ball as an individual property? Given this, is there then a way to calculate the coefficient of restitution in a collision between the two balls?