All our tables leave the factory pre-sealed.
Since we do not like to paint the surface of our lovely stones with transparent paints or resins, we apply three coats of the highest possible level of protective penetrating sealant. This will preserve both your table and the natural feel of the stone.
That said, please bear in mind that there is no sealer on the market that can make a natural stone 100% stain proof, especially if you leave the staining agent on it for a prolonged period. The sealant allows you to have a few hours between the moment the staining agent (wine, coffee, perfume, etc) comes in contact with the stone and the moment you wipe it off. So, if wine spills on your table, you will need to wipe it off within a few hours to avoid permanent staining.
As far as maintenance goes, DO NOT use cleaners containing abrasive cleansers, ammonia, bleach, acid or even those “homemade remedies” made from lemon, vinegar and the like. Simply use water or a neutral detergent. It will be more than enough.
If you are a particularly assiduous cleaner, you can set your mind at rest by periodically applying a new coat of sealer.
You can buy that here and then simply follow the instructions on the pack.