Solid fuel burns at excessive temperatures and for that reason can crack Marble & Granite. The excessive heat circulates around the back panel and is not drawn up the chimney, eventually the heat forms a fault on the Back panel and results in a crack to disperse the heat. The obvious solution the fireplace industry came up with to solve this problem was to create purpose built joints, and hence create an area for the heat to escape. Solid fuel back panels are always made in three sections as opposed to a solid one piece panel. Back panels also need to be made denser to absorb the intense heat generated. Although Marble back panels and granite back panels are made from 20mm sheets of stone, the panel would still crack even if 30mm to 40mm solid pieces were used. The solution to making the back panel more absorbent to heat is to create a cavity box and fill this with cement (please see image below).  The process of manufacturing back panels in three sections with cement filled at the back is commonly known as Slabbing.

Price guide (from creative Fireplaces website)

A Marble back panel and hearth cost £140 A Granite Back Panel and hearth £190 Slabbing adds £100 (additional) to the cost of a back panel and hearth