Limestone refers to natural stones that have been fashioned together, grain by grain, over thousands of years. While some limestone rocks, like certain marbles, are dense enough to resist staining, most are so porous that they easily absorb liquids. A prime concern for limestone stains are acidic solvents like lemon juice, which can also etch the underlying stone in a matter of minutes. Because it can be so easily stained, it is highly recommended that limestone tiles be sealed to prevent them from being ruined. Most experts also recommend against using limestone in a kitchen because of its strong reaction to acids.