How is tayammum performed with a stone, and how are the hands wiped if the stone is small?

It is permissible to perform tayammum with anything on the surface of the earth, such as stones. With a single strike, one wipes the face and the palms of the hands. Sheikh Ibn Baz holds that uttering "Bismillah" (in the name of Allah) and making remembrance thereafter is like performing wudu with water. If the stone is small, it is permissible to turn it over with the hands instead of striking it. It is preferable to perform tayammum with dust/earth if available, due to its precaution and validity according to all scholars.