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Islamic Laws : Obligatory acts relating to Namaz » Things on which Sajdah is allowed
Fatawa of Ayatullah al Uzama Syed Ali al-Husaini Seestani
1085. Sajdah should be performed on earth, and on those things which are not edible nor worn, and on things which grow from earth (e.g. wood and leaves of trees).
It is not permissible to perform Sajdah on things which are used as food or dress (e.g. wheat, barley and cotton etc.), or on things which are not considered to be parts of the earth (e.g. gold, silver, etc.). And in the situation of helplessness, asphalt and tar will have preference over other non-allowable things.
1092. Turbatul Husayn is the best thing for performing Sajdah. After it, there are earth, stone and grass, in order of priority.