What are the conditions of the earth with which one can perform valid Tayammum (dry ablution) in terms of type and size? Is it permissible to use a stone that has been burned? Is it permissible to use a stone that is not exposed to the sun rays? Thank you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

Maaliki and Hanafi scholars are of the view that it is permissible to perform Tayammum with a stone, provided that it is not changed to another substance through burning. If it is burned so that it is no longer customarily considered a stone, it is impermissible to use it for Tayammum. Al-Kharashi a Maaliki scholar, said: "It is permissible for both a sick as well as a healthy person who lacks water to perform Tayammum using a wall of mud bricks or stone, unless it is changed to lime or gypsum through burning or there is a barrier which prevents direct contact."