What makes tayammum unusual among Islamic ritual provisions is that its authorisation is not an interpretation later scholars derived from general principle, but an explicit textual concession stated twice in the Quran, at 4:43 and again at 5:6, both passages naming illness and the absence of water as the triggering conditions and both prescribing the same brief form, striking the hands on clean earth and wiping the face and forearms. That directness has not stopped the schools from developing genuinely different accounts of how far the concession extends.
The Hanafi school takes the broadest view of what counts as valid earth, extending it to anything belonging to the earth's substance, including stone, sand and lime, on the reasoning that the Arabic term used in the verses covers the ground generally rather than soil specifically, while the Shafii school requires dust fine enough to visibly cling to the skin, a narrower reading that would exclude bare rock with no dust on its surface. The schools likewise differ on how actively a traveller must search for water before resorting to tayammum, a question with real practical weight historically for travellers, soldiers and pilgrims crossing arid terrain where water sources were uncertain and searching for one could itself be dangerous or impractical, and modern fiqh councils have applied the same reasoning to situations the original desert context did not anticipate, including hospital patients for whom water contact is medically contraindicated and passengers on long flights with no ready access to water, extending a seventh-century desert concession into settings its original revelation could not have pictured.