The stone in granite countertops may contain a variety of minerals including calcite, particularly if the countertop is made from marble or limestone. Calcite reacts negatively to acidic foods like lemons or oranges, causing dulling or dark spots.
If possible, test the calcite content of your desired stone on a sample piece before purchasing. If your existing countertops are granite, select a small area that is not readily visible. Place a few drops of lemon juice on the stone. If it dulls or darkens immediately, sealing is required.
Purchase a commercial sealer for your granite countertop and use it according to manufacturer's directions. Use a cutting board to prepare acidic foods and place coasters under glasses. Avoid using cleaning products that contain lemon juice.