It is mindfulness which notices the distraction which has occurred, and it is mindfulness which redirects the attention to the stone. Mindfulness is more difficult to cultivate than concentration because it is a deeper-reaching function. Concentration is merely focusing of the mind, rather like a laser beam. It has the power to burn its way deep into the mind and illuminate what is there. But it does not understand what it sees.
Mindfulness can examine the mechanics of selfishness and understand what it sees.
Mindfulness can pierce the mystery of suffering and the mechanism of discomfort. Mindfulness can make you free. There is, however, another Catch-22. Mindfulness does not react to what it sees. It just sees and understands.