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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 will 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 (lithification). 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 may be 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.