Natural stone like granite is porous. Most granite countertops will retain moisture when they have not been properly sealed, so it is important to understand as well and to know how best to keep this from happening by using a sealer. Because granite countertops are so expensive, you want to make sure that you get the most for your money.
Why is sealing done?
The porosity of some stone types not only makes them retain water but they tend to stain quite easily. Sealing not only makes sure that your counter top is water resistant but it becomes stain resistant as well. Sealant however wears off, so this does not mean that the granite surface become water or stain proof because it has a sealant. Like wood, not all granite stones are the same or need to be sealed. The denser the stone the less likely it will need sealant. These stone types are more susceptible to staining. By sealing the stone you make it both moisture and stain resistant.
Resin treatment of Granite
Most commercial granite is treated with a resin made either from epoxy or polyester. This is done to fill in pits and blemishes that the stone might have naturally or gotten as a result of being cut. This process benefits the stone's overall resistance before it can even be polished. If a stone has not been resin treated it will be stained by almost anything, even water.
The misconception about dish soap
Another misconception is that of liquid dish soap. It should not be used to clean a granite countertop as it is likely to cause stains. You wouldn't expect that the very thing that makes dish soap an effective cleaner would affect the porosity of the stone itself. It is about chemistry. Soap is a chemically aggressive cleaning agent, aggressive enough to damage granite counter tops. What you use on granite or any natural stone countertop should be non-acidic, non-aggressive or abrasive for that matter. What it probably will do is wear out the sealant that is meant to keep the surface as water resistant as possible.