Those in the stone industry, and those interested in stone, should be familiar with basic geology.  As a doctor studies the systems of the human body to prescribe treatments and remedies, you should also be familiar with the formations and changes of stone. The study of these changes is known as geology.

Why is geology so important? There are many reasons. Granite and marble, for example, differ greatly in their hardness. A hard material such as granite may require harder abrasives for refinishing. Certain limestones contain very sharp minerals that can quickly damage a polishing pad or set of diamond abrasives. You need to know which stones these are.

The following is designed to provide you with a basic introduction to geology. In order to understand the problems you may encounter with stone, it is important that you understand the formations and changes that occur with stone. A thorough understanding will make your job much easier.

Stone Types
It is believed there are somewhere near 9,000 different varieties of stone. Of course it would be impossible for anyone to know every stone, but all stone can be classified into three distinct groups. Learning these three types and their characteristics can solve most of the problems you may face. For example: Granite, which is an igneous rock, contains quartz, which is very hard. For this reason, the proper abrasive must be chosen for finishing. Marble, which is a metamorphic rock, contains calcium carbonate that reacts to acid. This knowledge would tell you that acidic cleaners would damage most polished marble surfaces. Travertine, a sedimentary stone, consists of small grains of minerals bonded to each other with softer minerals. The minerals are very coarse and can rapidly damage a diamond abrasive. The following are the three classifications of stone and how to identify them.

Igneous Rocks (Granite)
Igneous rocks are formed from the solidification of magma deep in the earth. They contain 45 to 66 percent silica (quartz). The remaining minerals are mostly feldspar, mica and iron ores. Granite is the most abundant igneous rock found on Earth.

Characteristics of Igneous Rocks
Rocks exhibit a crystalline form with grain size ranging from very small to several inches. The large crystal granites are formed when the magma cools slowly. The smaller crystals are formed with rapid cooling. All this takes place before the magma reaches the Earth’s surface. If it does reach the surface, it’s called lava.

Hardness ranges from six and higher on the Mohs’ scale (see Figure 1). Igneous rocks can be identified very easily by performing a scratch test with an ordinary knife blade. If it is difficult to scratch, it is most likely igneous.

Igneous rocks will generally not react with acids. However Hydrofluoric Acid (HF) will react with a polished surface. Many stone cleaners designed for sandstone contain HF. Make sure to read product labels and material safety data sheets when using any stone cleaner or chemical. Avoid using HF on all stones if possible.

The minerals contained in igneous rocks are usually dense and packed tightly.

They lack bedding or foliation. However, be aware of a granite look-a-like known as gneiss. Gneiss looks like an igneous rock but is a transitional material between metamorphic and igneous. It is a brittle material and does have a bedding plane.

Color key for igneous rocks:

White granites are rich in potassium.
Combinations of mica and quartz result in gray.
Pink contains sodium and calcium rich feldspar.
Sedimentary Rocks (Limestones and Sandstones)
Sedimentary rocks are produced from erosion of other rocks, compression and underground water erosion. The sedimentary rocks cover nearly 75 percent of the Earth’s surface. For our purposes, sedimentary rocks can be classified into two types: limestones and sandstones.

Limestones are formed in shallow waters of the sea shelf. They consist of calcite, but maybe mixed with other minerals depending on the water clarity when it was formed. Many limestones contain a mineral known as magnesium carbonate (dolomite).

Dolomite does not react with acid unless it is first crushed. Any rock with more than 50 percent carbonate mineral is classified as a limestone. Some common limestones are travertine, slate, coral, shell stone or coquina. Limestones have a bedding plane, and because they are formed in the sea, they often contain fossils and shell fragments that make them more easily identifiable.

Sandstones are also sedimentary rocks that are primarily quartz minerals loosely cemented together with calcite, iron oxides and/or mud. Sandstones are frequently used as a dimensional building stone. Major cities throughout the world contain buildings made with sandstone. Some common sandstone is brown stone and shale. Sandstone can be recognized by its distinct sand-like appearance. It fractures very easily around the individual grains, and exhibits a distinct bedding plane.

Metamorphic Rocks (Marble And Slate)
Metamorphic rocks are limestones that have been exposed to high temperature and pressure over a long period of time. This change is known as metamorphoses and causes the minerals to go through a molten phase. For this reason, most marbles have distinct veins, swirls or bands, but there are exceptions. Light veins may contain quartz and feldspar, while dark veins may contain hornblend or biotite. All marbles will have calcite and/or dolomite and will react with acid.

Mineral Identification
In order to identify the types of stone you may encounter, you need to be able to identify the minerals in the rocks. Certain groups of rocks contain specific types of minerals. We know granites are igneous and contain quartz, marble is metamorphic and contains calcite, and sedimentary materials such as limestone can contain dolomite.

The minerals contained in the stones we deal with have some physical and chemical properties. Some of these properties can be tested and will tell us which mineral we are dealing with. The following are those properties:

Luster — Luster is the way a mineral reflects light from its surface. A mineral can be metallic, which means it looks shiny, or nonmetallic, which has no shiny reflection but is described as glassy or vitreous, pearly, silky, dull, greasy or soapy.

Hardness — Frederick Mobs in 1822 developed the hardness test which measures how resistant a mineral is to abrasion. We commonly call this the “scratch test.” Taking minerals and scratching one against the other developed the test. The one that scratched was softer then the one that caused the scratch. This test was continued until 10 minerals were discovered. The minerals were arranged in order of hardness and given a number. “1” was the softest and “10” was the hardest.

Cleavage — Cleavage is how the mineral breaks or fractures. Some minerals, when they break, will have one, two or three cleavages. This is perhaps the hardest property to determine and will require the use of a field microscope or hand lens.

Color — Color is an important clue to identifying certain minerals. Stone gets its color from many of these minerals. Some minerals only need to be present in very small amounts to lend color to a stone. For example, many of the red granites and marbles are the result of the mineral iron.

Acid Test — The acid test is an important test and can immediately tell you if you are dealing with a calcium-based stone. A simple drop of a dilute solution of hydrochloric acid will tell you if you have marble, limestone or granite. If the acid bubbles and/or fizzes, you have calcium. No reaction means there is no calcium and you may have granite or possibly a dolomitic limestone.

Those are the most common properties used to identify a mineral. Geologists and others may also use specific gravity; magnetism, electrical properties, optics and taste for further identification.

Other Stone Types
Marble, granite and limestone are the most common interior stone types that you will encounter. However, there are others. The following is a brief explanation of those types:

Slate – Slate can be found both indoors and our. It is a stone that has been metamorphosed from shale and consists of clay-like materials. It can be recognized by its sheet-like appearance and is found in earth-tone colors. Slate will rarely have a high polish unless a coating is placed on it. Slate can be a problem for the architect/designer. It contains high clay content, so it will often flake, spall and develop efflorescence easily. This is especially true in wet areas. Slate is best treated with a good quality silicone penetrating sealer.

Sandstone – Sandstone is a sedimentary material, which consists of sand crystals cemented together with natural clays. It is seldom seen as flooring material, but an architect or designer will run across a floor or two. Sandstone is very rough because of its sandy structure and is very abrasive on diamond discs. It is acid-resistant and rarely polished. Sandstone is very porous and should be sealed with a penetrating sealer.

Quartzite – Quartzite is also a rare flooring material but is gaining in popularity. It is metamorphosed quartz sand and is classified as sandstone, exhibiting those properties.

Shell Stone or Coquina – Shell stone or Coquina is a limestone composed of broken fragments of shell and coral. It is a porous sedimentary material and is easily identified by its shell or coral fragments. It should not be resurfaced because it is very abrasive stone and will tear diamond discs.

Flagstone/Blue Stone – Flagstone is a term given to almost all material cut into thin, irregular shapes. Found extensively on sidewalks, foyer entrances and pool decks, flagstone can be almost any stone material but is commonly the sandstone type known as blue stone. Blue stone gets its name from its distinct blue color. Flagstone should not be ground and should only be cleaned and sealed.

Onyx – Onyx is a type of marble, which has been formed by the deposits of cold solutions. It is a translucent material with veins running concentrically to one another. It is very expensive and can be found on table-tops and small pieces of furniture. It can be treated just like marble and takes a high shine.

Soapstone – Soapstone is one of the softest materials composed of talc. For this reason it makes an excellent carving material and can be found on fireplace surrounds and hearths. It is treated like marble and should be sealed.

Travertine – Travertine is a limestone formed in hot springs. Water movement causes the erosion of the stone, creating numerous holes of varying sizes. A polished travertine will have its holes filled with a color-matched Portland cement fill. These fillers do not take a high polish and therefore the overall polish is blotchy.