Iranian Muslim Prayer Stone: "Turbah"

Shia Muslims use a "turbah" in their daily prayers, believing that the Qur'an requires prostration upon the soil of the earth. A turbah is a small piece of soil or clay, often a clay tablet, symbolizing earth. While use of a turbah is not compulsory, it is highly recommended; Shiites place their foreheads on the turbah while praying. The most favored soil is that of Karbala, Iraq.

In the absence of soil, plants or items made from plants may be substituted. This provision has been extended to include the use of paper.

The turbah is wrapped inside a cloth to protect it from being damaged. When in use during prayer, the cloth is placed beneath the turbah to keep it clean and also as they pray while their forehead is on the turbah.
Source:
http://mecollectibles.com/turbah-cloth/1055-iraq-islam-shia-namaz-mohr-earth-soil-clay-tablet-turbah-cloth.html
http://en.wikipedia.org/wiki/Turbah