The World Federation One Stop Fiqh

Ruling 1046

There must not be a barrier between one’s forehead and the thing on which it is permitted to perform sajdah. Therefore, if the turbah37 is so dirty that his forehead does not make contact with the turbah itself, the sajdah is invalid. However, if, for example, only the colour of the turbah has changed, there is no problem.

37 A turbah is a piece of earth or clay on which one places his forehead in sajdah.