The hardness of a stone is usually indicated as a number which refers to the so-called MOHS table. This is a list of minerals, so ordered that a mineral can scratch the mineral before it in the table, but not the mineral after it.
Natural stones are compositions of different types of minerals and the concentration of these minerals can vary widely from tile to tile, block to block, etc. The hardness of a particular natural stone is therefore determined by taking the average hardness of the minerals which it contains (but without taking account of the relative quantities of these minerals). This means that the hardness figure is not always fully accurate for every piece of stone of that type, but it does give a good indication of its susceptibility to scratching. The following are examples of stone hardness.

• granite, gneiss: 6 to 7 (based on the presence of quartz and feldspars)
• basalt, gabbro: 5 to 6.5 (based on the presence of feldspar and hornblende)
• marble, limestone: 3 (based on the presence of calcite)

This means, for example, that marble and limestone can be scratched by an ordinary kitchen knife, but that granite cannot.