Welcome to stone 101. Here we will discuss the basic geology of natural stone. There are over 9,000 different stones in the world, and there are many variations from stone to stone which will directly effect the application of the stone. Granite, marble, and limestone are the three main stones that we will be dealing with, and in the following paragraphs you will find the basic composition and application of each of these stones.

Granite

Granite is classified as an igneous rock. Igneous rocks are formed from the solidification of magma deep in the earth. They contain 45 to 66% silica (Quartz). The remaining minerals are mostly feldspar, mica, and iron ores. Granite is the most abundant igneous rock found on earth. Igneous 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 when cooling is very rapid. All this takes place deep in the earth before the magma reaches the surface. If it does reach the surface then we have what we call lava. The hardness of igneous rock will range from 6 or higher on the Mohs scale of hardness (see bottom of page). The minerals contained in igneous rocks are usually dense and packed tight, and they will generally not react to acids. However, hydrofluoric acid (often found in rust removers) will react to polished surfaces, therefore you should always check any cleaners used to clean the stone. The scratch and stain resistance of granite makes it a perfect stone for kitchen countertops, high traffic areas, and outdoor applications.

Marble 

Marble is classified as a metamorphic rock. Metamorphic rocks are limestones that  have been exposed to high temperatures and high pressures over a long period of time. This change is known as metamorphosis, hence the name metamorphic. The change causes the minerals to go through a molten phase. For this reason marbles often have distinct swirls or bands. Limestones, and therefore marbles, are composed of the minerals calcite or dolomite. Marble in its purest state is white, colored marbles are the result of other minerals being mixed with the calcite or dolomite. Marble is relatively soft when compared to other stones, such as granite. It will scratch very easily, marble is also very sensitive to acidic chemicals. It is extremely important to keep any marble surface out of contact with the following acidic materials: lemon, tomato sauce, bleach, coffee, fruit juices, wine, urine, vomit, tile cleaners like Tile X, the various mildew removers, X-14, acidic toilet bowl cleaners, cleaners containing lemon, pool pH decrease (muriatic acid), driveway cleaners, and vinegar. Since marble is such a soft stone it is not recommended for use in high traffic areas. The most common application for marble is in the bathroom for vanity tops and tile flooring. Marble is also a good stone for columns and fireplace surrounds.

Limestone & Sandstone

Limestone is classified as a sedimentary rock. Sedimentary rocks are simply produced from erosion of other rocks as well as compression and underground water erosion (Lithification). The sedimentary group of rocks cover nearly 75% of the earths 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 how clear the water was 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% carbonate minerals is classified as a limestone. Some common limestones are travertine, slate, coral, shell stone or coquina. Travertine differs from other forms in that is formed in hot springs called karst. The water movement in these karst erodes the travertine, creating holes in the stone. Sandstones are also sedimentary rocks that are primarily quartz minerals loosely cemented together with calcite, iron oxides and/or mud. Sedimentary rocks are mainly used on walls, floors and building exteriors.

Mohs Scale of Hardness

          Softest               1. Talc

                                    2. Gypsum 

                                    3. Calcite

                                    4. Fluorite

                                    5. Apatite

                                    6. Orthoclase 

                                    7. Quartz

                                    8. Topaz

                                    9. Corundum

                Hardest       10. Diamond    

Granite falls in between 6 and 8.

Marble falls in between 3 and 5.