Use a good name brand such as Custom Building Products or Aquamix. The main types are penetrating sealers and top sealers. Penetrating sealers soak into the stone and do not normally change the appearance of the stone. Top sealers generally only work on the surface, enriches or darkens the color of the stone, and adds either a matte or gloss sheen.
Michael Prachniak - Expert Tile and Stone Consultant and Writer, Owner - Tile Productions
Check out FAF''s tile section!