Surface staining is where a chemical or substance has left a coloured mark on the surface. This stain/mark may be sitting on the surface or may have penetrated the stone – depending upon how porous the stone is. Sealers will buy you some time to protect against the offending chemical before the surface is permanently stained. This will vary depending on the sealer, stone and chemical involved – some chemicals can stain in minutes. Etching is a totally different problem. In many cases, stone surfaces are actually etched not stained.
Marble and limestone are composed of calcite, which is highly sensitive to the acids in everyday food items, which corrodes the surface (etching) damaging the polished or honed finish. Even acids in water can affect some stones, leaving a white ring from a glass left on a bench top. Etching can appear as a blotch or dull spot, have a lighter coloured (white) bleached appearance, feel rougher, or feel and look as if the surface is pitted. Severe etching can lead to the surface crumbling and flaking. Some stones like marble and limestone can also be porous and are susceptible to both staining and etching at the same time. Some stones such as sandstone and basalts are not receptive to a high-honed or polished finish due to their open grains. This can make them more susceptible to staining and etching and bacterial infestation.