The World Federation One Stop Fiqh

Ruling 681

The thing with which a person performs tayammum must be pure (ṭāhir); and based on obligatory precaution it must also be commonly considered to be clean, meaning that it must not be tainted with anything that causes disgust. If one does not have a pure thing with which tayammum can be validly performed, then performing prayers at that time is not obligatory on him; however, he must make up that prayer; and it is better that he also performs the prayer within its time except if the situation has reached the point whereby he must perform tayammum with a dusty carpet or a similar thing.84 And if it is impure, the obligatory precaution is that he must perform tayammum with it, perform the prayer, and afterwards make up the prayer.

84 See Ruling 675.