One must perform sajdah on earth and on those things that grow from the earth but are neither edible nor worn, such as wood and the leaves of trees. It is not permitted to perform sajdah on edible things, nor on things that are worn, such as wheat, barley, and cotton. Furthermore, it is not permitted to perform sajdah on things that are not considered parts of the earth, such as gold, silver, and suchlike.
However, when one is compelled, performing sajdah on tar and asphalt (which is a lower grade of tar) take precedence over other things on which it is not permitted to perform sajdah.