This course is part of the Natural Stone Academy

Natural stone refers to stone that is quarried from the earth. Though it may be cut into shapes, including slabs and tiles, and a resin or sealer may be applied to its face, the internal fabric of the stone is unchanged.

It is important to understand what natural stone is not. Engineered products that are made from aggregates bound together with cementitious or resin binders do not meet the definition of natural stone.

Prized for its beauty, strength, and longevity, natural stone is one of the oldest building materials, and its applications are limited only by the imagination. Inside, stone can be installed as flooring and countertops, as well as in vertical applications such as wall cladding. Other options include mantels, fireplace surrounds, bath surrounds, and furniture.

All images courtesy of Natural Stone Institute

When a stone is tested for compressive strength, a ram is used to crush the stone until it fails.

Stone has just as many exterior applications. It is used extensively in landscaping, serving as pavers, walkways, walls, and other structures. Historically, stone served as the structure for many buildings; today, it is more typically used as non-structural cladding.

Dimension stone is stone that has been cut or sawn into specific shapes. How dimension stone is finished—whether honed, polished, or flamed—affects the stone’s appearance. Many types of natural stone, including granite, marble, and limestone, can be fabricated into thin panels and used on the exterior of buildings.

How Natural Stone Is Classified

When specifying natural stone, it is important to understand the material’s inherent characteristics so that you make an appropriate choice for the application. Knowing about the stone’s chemistry as well as how it was formed provides some basic information about how the stone will perform.

Natural stone can be divided into two broad categories based on its chemistry, or mineral composition. Calcareous stones are made of calcium carbonate and tend to be vulnerable to acids, even mild ones like lemon juice. Common types of calcareous stones include marble, limestone, travertine, onyx, and occasionally types of sandstone.

Siliceous stones are made of silica or silicates. These minerals make up close to 95 percent of the earth’s crust and include such common minerals as quartz, feldspar, and mica. Siliceous stones tend to be strong and resistant to acids; examples of siliceous stones include slate, quartzite, granite, and most types of sandstone.

Natural stone also can be categorized by how it is formed. Sedimentary stones are formed from preexisting rocks or the bodies of once-living marine organisms. Many sedimentary stones start out as sediments carried by rivers or glaciers that are deposited in lakes or oceans. The particles are “stacked” layer by layer; when trapped and buried, the particles become compacted and cement together. Common examples of sedimentary stones include sandstone and limestone.

Metamorphic stones are created through a combination of heat, pressure, and time. Metamorphic stones start out as sedimentary, igneous, or earlier metamorphic stones, but over a long period of time and under intense heat and pressure the crystals rearrange themselves or react with fluids that enter the rocks. Metamorphic stones, which include such common varieties as schist, gneiss, quartzite, and marble are usually denser and more compact than their earlier forms.

Natural stone comes in a variety of colors and patterns that tell us the story of the stone’s origins.

Igneous stones have their start as molten material that originates from deep within the earth near active plate boundaries. The lava is exposed to great heat and pressure, then cools as it rises to the surface. Lava that cools very slowly forms large grains, while igneous stones that cool quickly have a fine-grained or glassy texture. Igneous stones include gabbro and basalt; many of these are included in the granite group because they perform similarly to granites.

This brings up an important point. The stone industry uses commercial definitions for dimension stone types. Although these certainly overlap with scientific and geologic definitions, there are some differences.

The scientific descriptions focus primarily on a stone’s geographic locality and mineralogical composition, while commercial descriptions focus on workability and general performance of the stone. Hence, the commercial industry groups stones according to those which behave and perform similarly. While the stone industry recognizes all rock types, they consolidate them into just 10 groupings of stones: granite, limestone, marble, onyx, quartzite, sandstone, serpentine, slate, soapstone, and travertine.

We can use the two categories of chemical composition (calcareous and siliceous) and the three categories of formation (sedimentary, metamorphic, and igneous) to create a matrix to help organize the various stone types. Stones within each category have properties that are generally predictable. Later in this course, we will see how the design professional can use this matrix to create a plan to guide future specifications.

All stones are unique; consequently, expect variation in the properties and behavior between stone products, even if they are of the same type. While this variation makes stone an appealing and stimulating material, it also adds challenge. Research is critical, as the specifier must understand a stone’s strengths, vulnerabilities, and limitations.

There are several factors to consider when specifying natural stone. The first and most obvious is aesthetics. Will the stone help achieve the desired look? A stone’s aesthetic is determined by its inherent properties—color, grain size, and veining, for example—but also by how it is cut and finished. For example, a highly honed or polished finish will impart a more refined look.

Performance is just as important as aesthetics. Will the stone perform adequately for the desired application? For example, if the stone is to be used as a countertop, will it resist staining and hold up to abuse from steel knives? If installed on a building exterior, will it hold up to the elements?

It is also important to choose stone that is available in the dimensions and quantities desired, and within the allotted timeframe. Since by its nature stone is a heavy material, it is preferable to source it locally or regionally, although this is not always possible. Some highly desirable stone types—certain marbles, for instance—are only found in a few places in the world. In another example, most travertine used in the United States used to be imported almost exclusively from Italy and a few other countries.

Finally, cost is a factor. Even if the stone is the perfect material from an aesthetic and performance standpoint, it may not fit within the project’s budget.

Though all of these considerations are important, this course focuses on performance, and the design professional’s role in obtaining and interpreting information about how a given material will perform.