Characteristics of the common building stones Below is a table showing some of the important characteristics of the common building stones.  Stone type  Porosity  Absorption  Effect of Acid  Hardness* Composition   Colours  MARBLE  0.5 - 2%  Absorbs water and oils very easily  Very sensitive  3 - 5.5  Calcite plus veining minerals  From white through most colours with veining due to presence of coloured minerals  GRANITE  0.2 - 1.5%  Absorbs water very slowly  Resistant  5 - 7  Feldspar, quartz and mica  Many colours and patterns  LIMESTONE  0.6 - 31%  Absorbs water and oils readily  Very sensitive  2 - 5  Calcite  Generally pastel colours but can be quite dark  SANDSTONE  0.6 - 35%  Absorbs water and oils readily  Some can be sensitive  4 - 7.5  Quartz, feldspar and mica  White to earthy colours  SLATE  0.4 - 5%  Absorption depends upon hardness but does absorb water and oils  Hardly effected  2.5 - 5.5  Silicates plus other minerals  Most colours but commonly black or grey   *Hardness ..... this scale is based on Mohs scale of hardness, shown below;  HARDNESS  MINERAL  EVERYDAY MATERIALS  1  Talc  Baby powder  2  Gypsum  Fingernails  3  Calcite  Bronze  4  Fluorite  Iron  5  Apatite  Glass  6  Feldspar  Nail file  7  Quartz  Good quality Steel  8  Topaz  Sandpaper  9  Ruby Emeralds   10  Diamond  Industrial diamonds Importance of this hardness factor with regard to building stones is their resistance to scratching. When for example marble is polished it is ground down using diamond cutters which are used in ever finer grinders similar in principle to grades of sandpaper. This is attempting to grind the surface and the crystals as flat as possible. A very flat surface will reflect the light uniformly and look glassy or highly polished. As this surface is scratched it reflects the light less uniformly and therefore loses its shine. Dust has a hardness value of between 5 and 7 when it originates from other than skin! If we consider stone tiles laid as flooring then this will obviously be walked over. Shoes will deposit dirt on the surface. This dirt is made up of small particles of quartz plus other materials. As this is then moved around by the movement of other shoes it will scratch into the surface of the marble. (Quartz has a hardness of 7, marble only 5). As this process proceeds the floor will gradually lose its shine. The worst effects will be near the entrance gradually decreasing away from the entrance. Entrances suffer the fastest deterioration in their lustre but all areas that are walked on frequently will eventually decline. Even a simple process of dusting a surface will cause scratching which over time will reduce the shine. Vacuuming marble and terrazzo floors can also cause scratching from the vacuum heads. As a result it is best to dust mop such floors. Other problems with floors and surfaces made of natural stone other than granite is the effect of acids. Acids effectively dissolve the stone thus causing defects in the surface which will become dull and patchy. Many common household items are acidic, from lemon juice to coke. All of which if dropped on the surface even as small splashes will damage the surface and cause it to look stained. The only way of retrieving the situation is to regrind and polish. As a result marble, limestone and sandstone should not be used as kitchen worktops. Because of the porous nature of stone, coloured liquids dropped onto stone will enter the pores, the liquid element will eventually evaporate leaving behind the colouring as a stain in the stone. If the stone is impregnated with a suitable sealer then the liquid cannot get into the pores as quickly thus no staining should result if cleared away reasonably quickly.