Sealing natural stone surfaces is a decision that depends on where and how the stone is used. Sealants on newly fabricated natural stone surfaces can work effectively to keep moisture and food/oil/stains from penetrating.
Here are three major types of sealants available for newly fabricated stone:
Generally made from polyurethanes or acrylics. These sealers may be effective at stopping stains but, being exposed on the surface of the material, they tend to wear out relatively quickly, especially on high-traffic areas of flooring. This type of sealer will significantly change the look and slip resistance of the surface, especially when it is wet.
The most penetrating sealers use siliconates, fluoro-polymers and siloxanes, which repel liquids. These sealers penetrate the surface of the stone enough to anchor the material to the surface. They are generally longer lasting than topical sealers and often do not substantially alter the look of the stone, but still can change the slip characteristics of the surface and do wear relatively quickly.
Uses silanes or modified silanes. These are a type of penetrating sealer, which penetrate deeply into the material, impregnating it with molecules which bond to the capillary pores and repels water and / or oils from within the material. Some modified silane sealers impregnate deeply enough to protect against salt attack, such as efflorescence and freeze-thaw spalling.
Exterior reclaimed stone, such as antique cobblestone from the Historic European Cobblestone collection, does not require any sealants. Old reclaimed materials have already been exposed to all kinds of weather and dirt. After centuries of use the patina that exists will only get better. Nothing more than an occasional power wash is all that is needed to remove almost anything it will encounter. The worn patina is what makes this old cobblestone full of charm and value!
Consult your installer, stone supplier or sealant representative for your specific stone type. In general, if its newly fabricated and you walk on it, or water can get to it, you should probably consider applying a sealant.