Even ignoring the lack of moss on non-rolling stones (see below), I have serious doubts about the accuracy of the old saying “A Rolling Stone Gathers No Moss”. How long will a stone roll for anyway? All things considered, it would be hard for a stone to roll for more than a few hours. Furthermore, when the average life of a rock is considered, this would be a rather small percentage of a stones life. Even given a short life of 10,000 years, and a rolling time of a week, this would be only a mere 0.000001916% of it’s life. I feel that the saying was just some made up to discourage people living a nomadic lifestyle, or perhaps to discourage change. Obviously too little effort was put into its creation, and no thought was given to the image of stones in general.
Stone A was given to my daughter who rolled the stone around for a week. No moss gathered.
Stone B has been in my garden for a few years and in that time no moss has been detected.
Stone C was left next to a pond for a month to attract moss. No moss has been noticed but there might be some under the frog. Further tests will be conducted.