It's a rock. That's right...it's just a rock.
And why on earth is a coach, who claims to be a Christian, be rubbing a rock for good luck? Isn't that sort of a throw back to the superstitions of the dark ages?
You can't say it's all in fun, while at the same time attaching a huge monetary value to it. If it's all in fun, then just glue the piece back on or forget it ever happened.