Because stone is the result of a natural process, variations exist in…

HARDNESS , DENSITY ,POROSITY , PERMEABILITY , ABSORPTION , DURABILITY,  WEATHERING ,VEINING , COLOR
These individual characteristics also have an important impact on the performance of a stone and developing a successful stone maintenance and preservation program.
HARDNESS / DENSITY
Hardness and density in relation to stone surfaces is a measure of the resistance of any substance that will scratch or abrade its surface. The hardness factor is a physical property of the stones mineral composition, density and porosity. Different minerals and stones have differing degrees of hardness and density and therefore have differing degrees of resistance to scratching and abrasion.

The Mohs’ Scale of Hardness is the method used to measure the ability of one substance to scratch another. The scale ranges in order of increasing relative hardness from 1 (softest) to 10 (hardest).
 

ABRASIVE HARDNESS
Abrasive hardness (Ha) is an industry method to determine the ability of a particular stones resistance to wearing from abrasive substances. This can be an important consideration when selecting stone for flooring in a commercial setting.
The American Society for Testing and Materials (ASTM) has set values for abrasion resistance and the stones Ha rating can generally be ascertained by requesting a Technical Information Sheet on the particular stone in question from the supplier. If this is not available a sample of the stone can be sent to ASTM requesting a C241 evaluation be preformed.
The Marble Institute of America recommends that stone being considered for flooring have a Ha of 10 or more and the ASTM recommends a Ha of 12 or more for stone flooring in heavy pedestrian traffic areas. It is also recommended that if different types and varieties of stones are used together that the Ha ratings should not be to far apart from each other. This will ensure a uniform appearance when the stones begin to show wear.

The dirt and grit that is tracked into buildings have a hardness range from 2 to 7, and foot traffic tends to have an abrasion factor of 120 to 220 grit. This is equivalent to that of  andpaper at the same grits.