SHAFAQNA – The Grand Ayatollah Sistani answered a question about prostrating on the stones of Masjidul Haram.
The Question: Is it allowed to prostrate on the stones of Masjidul Haram considering that they return heat and are not affected by the sun which make them different from other stones and it is said they are artificial stones and are not natural?
The Grand Ayatollah Sistani: If the stone made of artificial substances, are of types which make prostrating on it correct or those substances have been worn away from it, (these) are not obstacles for prostrating. This is at the situation of non-Taqiyyeh but in the case of Taqqiyyeh, prostrating on it is allowed, even if it is made of something that prostration is not allowed on it.
Source: PERSIAN SHAFAQNA