Among flooring materials, none is more elegant and luxurious than natural stone. The term “natural stone” refers to a variety of mountain-born mineral substances that stand in contrast to any synthetic or manufactured stone products. Common natural stone flooring includes slate, marble, limestone, travertine, granite, and sandstone—each of which has slightly different properties. It’s important to understand the characteristics of the type of stone flooring tile you are purchasing in order to determine whether it is appropriate for a specific location.

Absorption Rating

The absorption rating refers to how porous a given material is. The more absorbent it is, the more susceptible it will be to stains, as well as cracking damage when subjected to freezing conditions. Natural stones vary greatly in their absorption rates, with sandstone being the most porous to granite, which is virtually waterproof even when left unsealed. Absorption rate will be classified according the following terms: 

  • Non-vitreous: This is the highest absorption level. In most cases, non-vitreous tiles should not be used in any damp environment. 
  • Semi-vitreous: While these tiles are less absorbent, the more liquid they are exposed to, the more maintenance they will require.
  • Vitreous: This is the standard absorption level for flooring tiles and these materials are generally considered appropriate for most low to mid traffic indoor and outdoor applications.
 
  • Impervious: These materials are resistant to the absorption of liquids and thus will be easier to maintain. They are often used in high-traffic commercial applications.

In general, sandstone is the most porous natural stone material. Travertine, limestone, and slate have medium absorbency, while granite is relatively waterproof.

 

Polished materials also absorb less water than honed or cleft surfaces.​

Grade

Some retailers use a grading system to rate the quality of materials. This can refer to the size, shape, and thickness of the tile, as well as the condition of its surface. Most grading systems have three levels of quality.

  • Grade 1 refers to high quality, uniform materials.
  • Grade 2 consists of materials with minor defects, such as chips, scratches, or irregular surfaces.
  • Grade 3 materials have major flaws in size, shape, surface, or chipping, making them appropriate only as accent pieces, or in certain rustic decorative applications.

Coefficient of Friction

This measures how slippery various materials are. The higher the coefficient, the more traction a tile will have. This number is especially important in moist environments such as bathrooms and kitchens, as well as high- traffic commercial areas. The Americans with Disabilities Act requires that flooring material has a minimum of a .6 dryness coefficient.

Indoor vs. Outdoor Rating

Some natural stone flooring materials are more suited to outdoor applications than others. Many of the factors above will determine whether a material should be used in an open air environment.

 

Non-vitreous materials will be subject to staining through dirt and acid rain, as well as cracking when absorbed materials freeze and expand. Stones which have a low coefficient of friction will also pose a slipping hazard during rain and snow storms. 

Oxidation

Natural stone materials are formed beneath the earth over millions of years, and often contain a variety of disparate elements. Sometimes iron is present in these materials, which can manifest as bright red and amber hues in the surface of the stone. The problem in an outdoor environment is that those traces of iron can oxidize, a process more commonly known as rusting. This can cause the entire tile to degenerate over time.​

Benefits of Using Natural Stone Flooring

There are many aesthetic and practical reasons why natural stone flooring can be a good choice: 

  • Each piece of stone is a unique creation of the earth, making every flooring application one of a kind. Every floor is entirely unique.
  • The mountain-born qualities of the stone can help to give living spaces a direct and eternal connection to the natural world, unlike any other building material. 
  • While there is some debate about the ecological impact of quarrying and transporting stone materials, the tiles themselves are natural, nonpolluting, eco-friendly pieces. Purchasing stones which were acquired locally can cut down on the environmental impact of transport.

Drawbacks of Using Natural Stone Flooring

There are also some drawbacks to the use of natural stone: 

  • With the exception of granite and some slate, natural stone is quite porous and needs to be treated with a sealing agent periodically to protect its surfaces.
  • Some polished materials such as marble can scratch easily.
  • Some stones are also very brittle and will chip easily.

When purchasing natural stone flooring materials it is important to do your research and understand the characteristics of the material you are purchasing. Find out whether it is appropriate for your specific application and how much maintenance it will require. Ask your retailer multiple questions, and get to know the material as much as you can before you make a purchase.