As one of the "older" players, I have also identified the darker ball as the mud ball.
What the cue ball manufacturers did was try to improve on the ball that had the solid metal core. The problem with it was that the metal would shift after a few hundred breaks. After that the ball would roll off so bad that it had to be replaced. So they came up with the mud ball you speak of, which has the metal chopped up and distributed throughout the whole ball. That is what gives it the darker color. It turns out that it was no improvement at all because most players hate the thing. It just reacted very differently.
As for the big rock, it is simply larger than the other balls.