Patio stone can be divided into two categories. Siliceous stone (granite, slate, sandstone, brownstone, and bluestone) is hard and durable. Calcareous stone (marble, travertine, and limestone) is more delicate and porous, so it is susceptible to stains. To tell which kind you have, put a drop of white vinegar in an inconspicuous spot. If it fizzes, the stone is calcareous. To clean, use a pH-neutral stone cleaner, and scrub with a soft-bristle brush or broom. Avoid household detergents, which tend to be too acidic or too alkaline. Repeated use of them can erode calcareous stone and dull the polish on siliceous stone.
To remove stains, use a stone poultice. Read the label to be certain you're choosing the right stain remover for your needs. When cleaning siliceous stone, you can boost the power of a stone poultice by adding a little all-purpose household cleaner, as long as it doesn't contain chlorine bleach.
To remove moss and organic matter from siliceous stone, use a stone cleaner that has phosphoric acid. For calcareous stone, choose a nonacidic algae remover. Weathering and the sun will fade most stains over time, but they fade the luster of surfaces, too. A stone enhancer will bring a more saturated look to the stone, particularly to slate and tumbled marble. If you opt for an enhancer, choose one with a built-in sealer to add stain resistance.
Calcareous stone surfaces in high-traffic areas will benefit from sealing to aid stain resistance. Remember that the sealer won't make surfaces stain-proof. Choose a penetrating stone sealer, which allows vapor transmission to prevent damage from freezing and thawing. As always, read labels to be sure the sealer is designed to protect against the stains you're likely to encounter. Reapply it every three to five years.