Granite is a natural stone that is used as counter and vanity tops in many kitchens and bathrooms. This stone comes in a wide variety of colors and styles, some of which are light in color. No two granites are exactly alike, which means that some stones may be more likely to darken on contact with water. Learning why this happens also can help you to take steps to stop it.
The Granite Surface
Granite is the term given to any commercially offered crystalline stone. In fact, not every stone sold as granite actually is granite. Some stones may be gneiss or gabbro, rather than true granite. What this means is that while each of these stones will contain some level of mica, silicone, quartz and feldspar, they are put together chemically differently from one another. Some dark colored stones will be extremely dense, with a tightly woven crystalline structure. Other light colored stones will have a much more open structure on their surface made up of thousands of tiny cracks and pores invisible to the naked eye.
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Some granite vanity tops are extremely porous, and are covered in microscopic holes, pits and cracks. When water or another liquid is left on top of the granite, small amounts of it seep into the stone through its many pores. When the surface spill of water is wiped away, the stone appears darker where the water was standing. What you are seeing is the water trapped inside the granite, filling up these holes. The water saturated stone will look darker until the water evaporates and leaves the stone behind.
While most water spots and dark areas do eventually lighten and leave the stone, this is not always the case. Water that contains high amounts of iron, for example, may deposit this mineral inside the stone. This permanently darkens and discolors the granite because the mineral will not evaporate with the water. Other liquids that can stain the surface of the stone permanently are oils, including fingertip oils, as well as soap, wine, coffee, ink, hair dye and foods, such as blueberries or tomatoes. Any liquid can penetrate and either temporarily or permanently darken the stone.
Stopping the Darkening
Water that temporarily darkens the top of a granite vanity top is a good indicator that the stone is a candidate for sealing. Sealers are water or silicone-based polymers that penetrate the tiny pores in the stone and fill them invisibly. This causes water and other liquids to remain on the surface of the granite, rather than seeping inside of it. Sealers do break down over time, particularly if they have been in contact with acids or some cleaners. When a well sealed stone stops beading water off of the surface and begins to darken, it is time to reseal the stone.