Selecting Natural Stone 

By Russ Magnuson

Stone is a natural product. But perhaps more important, it is a product which is still in a state of nature. Its performance as a building material, whether in tile, slab or any other form, is entirely determined by the materials that comprise it and the natural forces that have brought it into being. For this reason alone, it is not just interesting but essential to understand the characteristics of any stone you intend to use for any purpose.

The general term for this material is dimensional stone, natural stone or, most simply, stone. You will often hear any polished stone referred to as marble. This is not always correct. Misidentification of a particular stone can pose problems, especially during the maintenance process. It is not important that you know the precise commercial name of a particular stone; there are more than 8,000 varieties, so being absolutely sure of that can be difficult. What is critical is that you know what type of stone you are dealing with.

Basic Stone Classifications Most of the stone used today falls under one of three major classifications. All three have to do with the origins of the stone, that is, how it was formed.

The first classification is igneous. This literally means formed from fire. An igneous stone was once a molten, flowing mass. All of its impurities were burned out, and as it cooled it became very dense and hard. The material that is commonly referred to as granite falls under this classification.

The next stone classification is sedimentary. This material was formed as a result of minerals and clays being deposited in such areas as old river bottoms or flood plains. Over time and through very little mechanical action, this mixture solidified. If pressure and heat were applied, this mass would be changed into marble.

Sedimentary stone is generally more porous than marble or granite. It is also much less dense. Limestone and sandstone are the most common representatives of this category.

The third classification is metamorphic. The term denotes that it has undergone a change. Stone of this type is formed of a mixture of clays, carbonates and other minerals which has changed under intense pressure and heat inside the earth. A crystalline structure has typically been formed. Marble is the most common example of a metamorphic stone. Stone in this category tends to be more porous and less dense than igneous stone.

These three major classifications can be broken down into groupings or types of stone whose names are much more familiar to us. Granite, marble, travertine, slate, limestone and sandstone are types of stone that we run across on a regular basis. Here is a brief and very basic guide to the most commonly found types of natural stone. Please bear in mind that these are not exact, geological definitions; they are the ones actually used in stone trades.

Granite Granite has a noticeably granular, crystalline structure. The term derives from the Italian word granito, or small grain. The grains can be consistent in size or they could vary. They could range from the size of a BB (2mm) to the size of a quarter (25mm). Granite is comprised of feldspar and quartz. It can also contain micas and hornblende.

Granite was formed from molten materials. Its source is magma, which is still escaping from the core of the earth in the form of lava. What makes this mixture of feldspar, quartz and silica different from lava is that it has cooled very slowly, under extreme pressure, deep within the earth.

The more slowly the magna has cooled, the finer the resulting crystal structure or grain. This cooling process, which has taken millions of years, has produced a stone which is extremely dense and hard. In fact, it is second only to diamonds in hardness.

This igneous stone is very consistent in appearance. Each square foot looks almost like the next. There are generally no veins present. There are some types, Juparana is an example, which seems to show a band of dark color which can look like a vein. Upon closer examination, this vein will be seen to be granular in nature. It will be much wider than a quartz or oxide vein found in marble.

Granites which were allowed to cool slowly, without interruption, are very consistent. But if earthquakes, continental plate movement or glacial activity have disrupted these formations, they were moved around and reheated. Impurities fell into tiny fissures and produced a wavy or veiny look.

Granites are quarried in many countries around the world. We have deposits of granite in the U.S. It can also be found in Canada, South America, Scandinavia, Italy, India and China.

Granite can be finished in several ways. The surface can be highly polished; this means it will reflect a great deal of light. It can be honed to exhibit little if any light reflection, and to be more slip-resistant. Or it can be given a flamed or thermal finish, which is achieved by running gas jets (flame) over the surface of the stone. The very high temperatures produced by the burning gas (3,100-3,600 degree F) cause a rapid evaporation of moisture from within the stone, causing crystals at the surface to explode. The resulting surface is heavily textured. A flamed or thermal finished surface will offer the best traction for walking. It is very slip-resistant and is often found in commercial lobbies or on exterior walkways.

Granite is very dense and hard. It stands up well to heavy foot traffic, which makes it the stone of choice for commercial lobbies and walkways. It is resistant to most chemicals, although oils can permeate into it. Granite is ideal for counter and bar tops. Some of the very light grey to white granites are somewhat more porous than most other types. White granite will contain tiny flecks of iron; these can pose a problem with rusting when used on exteriors or in wet areas.

Marble Marble may be a combination of different carbonate materials. Composed primarily of calcium carbonate or calcite, it can also contain calcium and magnesium or dolomite.

This metamorphic stone generally has a consistent background with veins of different colors. There will also be tiny fissures or cracks showing at the surface. If these fissures are open, or appear as a depression in the surface, they will have been filled during the fabrication process. They can also be filled during the restoration process.

Marble is formed in a different way than granite. Marble has never undergone a molten stage. It is a sedimentary material that has been recrystallized under extreme pressure and heat. Marble is basically pressed together; it has never flowed like granite. Over time, earth movements have exerted stresses on the stone. Minerals and other impurities have found their way into the resulting cracks or fissures to produce the veins which we see in marble today.

Marble is available in a wide range of colors. No two pieces are exactly alike. Much of the beauty of marble derived from this tendency toward endless variation.

Marble is quarried in most countries of the world. We have several marble quarries in the U.S., and the stone also can be found throughout the rest of North, Central and South America. It is very common in Italy and Greece, as well as China, Southeast Asia and India.

Although marble is generally given a highly polished surface, it is becoming more popular in the U.S. to use marble with a honed surface. This improves the slip-resistance factor of the stone. Honed finishes are used throughout the world on heavily trafficked floors. This type of finish hides trafficked floors. This type of finishing hides the scratching that shows on a polished surface. This type of stone is more porous than granite. It is also softer, so when it is used in highly polished form, it will show wear sooner. This is a natural occurrence, and the surface can be restored.

Limestone Like marble, limestone is made up of calcium carbonate or calcium and magnesium. However, limestone is a sedimentary material formed near the surface of the earth , it has not undergone a change or metamorphosis. Thus it does not have a visible crystalline structure; you can’t see into the stone. It tends to be fine grained.

In color, this stone is generally light beige to a rosy brown. It is commonly given a honed finish. Most limestones do not accept a high polish, although there are some from Germany which will polish very well. The surface of limestone contains a myriad of tiny voids or holes. When the surface is brought up to a highly light reflective stage, these holes are accentuated, making the resulting polish look blotchy.

Limestone has been used for hundreds of years as a building material. Many older buildings are clad in limestone and have very durable flooring made of this sedimentary stone. Limestone is undergoing a revival in popularity and is once again a favored material for floors, walkways, walls and countertops throughout the U.S.

Limestone tends to be porous, although some varieties are much lower in porosity than others. It is susceptible to staining. Sealers and harsh chemicals will greatly affect the look of the stone.

Travertine Travertine is a type of limestone. Calcium carbonate is its principal constituent. Its distinctive characteristic is the pores or holes that form a pattern across its face. The pores have been formed over time by escaping gases or very hot liquids venting from the interior of the earth to the surface; it is sometimes said that "travertine was once a river bed."

This stone can be finished in a number of ways. It can be left in its natural state that is with a coarse surface with the holes exposed. The surface can be polished, leaving holes open, or the holes can be filled in with either a cementitious or a polyester resin fill. Either can be tinted to match the predominant color in the stone. The polyester resin fill will be shiny when it dries, while the cementitious type will always be dull.

Travertine will vary from almost-white to beige. It can also be found in gold-to-brown tones, and there are gray and red travertine. It is a durable and commonly used material which sees extensive use for floors, walls and table tops.

Slate Slate is metamorphosed shale. It is composed of mica, chlorite and quartz. Its structure is layered or stratified. This stone is generally split along stratifications and cut to size.

The surface of slate is generally uneven. This is due to the cleaving or breaking of stone along its layers. It is found in a range of colors, or which gray, black and green are the most common. We are now seeing red, yellow, brown and purple slates on the market. Most of these are variegated colors, which can make for a very striking floor.

Slate is a very dense material whose low porosity allows it to be used effectively in exterior applications.

Agglomerates Agglomerates are man-made products. Chips and pieces of marble or granite are mixed together with polyester resin to form a block or slab, which is then cut and finished. The material can be polished, honed or textured.

This material can look very much like natural stone. It is manufactured in a rainbow of colors. You can find it in use anywhere that is appropriate for natural stone. Most agglomerate stones are polished. However, those agglomerates which are composed of granite will generally be given a honed finish. Agglomerate stone is a unique product; from a maintenance standpoint, it is best to treat it as a marble.

The Porosity Factor While there may be many variables between the different types of stone, there are some common characteristics. The most important is the fact that all stone is porous. Sedimentary stone is more porous than metamorphic, which, in turn is more porous than igneous; but all are porous to some degree.

This porosity or permeability leads to a natural process that occurs in all stone: vapor - transmission, the passing of moisture in gaseous form through stone. Breathing is a term commonly given to this phenomenon. You may have heard it said that stone must "breathe." This means that moisture vapor must pass back and forth through the stone. It will happen, and cannot be stopped.

If you try to slow down or stop this natural process, changes will occur within the stone. It will begin to darken; if moisture is trapped within the stone, it will begin to react with calcium, lime and oxides contained in the stone or the setting bed. A white look will appear in areas. The term for this is efflorescence. In areas subject to extreme moisture, a decomposition of the structure of the stone itself can begin.

The descriptions provided in this article are very general. There are many more stone types and classifications, and specific stones can vary greatly from the general description of categories under which they fall. However, the stone types discussed here are the ones you are most likely to encounter. It is not as important to know the exact commercial name of the stone you are dealing with as to know what type of stone it is. This will alert you to some of its weaknesses, and where to place particular emphasis during the all-important process of maintaining it.

Reprinted from "Tile Design & Installation" - November 1995

Russ Magnuson has been involved with natural stone since 1977. He has extensive experience in sales, installation, fabrication and restoration, and currently serves as technical service manager for restoration and maintenance products with Vic International Corporation.

This article is adapted from a chapter of Mr. Magnuson’s forthcoming book, How Do I Clean My Marble? The Complete Guide to Stone Maintenance.

NATURAL STONE SAMPLE APPROVAL:

All stones are a product of Nature. Variations occur from one piece to another, and as well as within each stone. All procedures will be taken to allow for blending required to meet owner/architect/builder approval prior to installation. Absolutely no claims will be accepted for any reason after materials are installed. Both owner/architect/builder will be required to sign an approved sample stone.

 

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