Granite: What is it?

In recent times the term granite has been used to describe an ever increasing range of stone. However are these all “real” granites or are some of them inferior look-a-likes? This is a question I get asked constantly. To properly answer this question we must also pose another question and that is: Is there a practical and significant difference in performance between so called “real” granite and those that simply use the term for convenience?

I split granite into two groups, geological granite and commercial granite. Geological granite is the stone the term “real” granite refers to and the remaining granites fit into the commercial grouping. Granite is an igneous rock which means it is made by a combination of heat and pressure. It is made up of primarily four minerals, quartz, feldspar, mica and usually hornblende. 

Quartz: quartz is a common mineral found in many different types of rock. The different colours of quartz are due to impurities mixed at the time of formation.

Feldspar: Feldspar is the most common and abundant mineral in rocks that are formed on or near the surface of the earths crust. Feldspars can be various colours from white to blue and even green and red. They are easily weathered when exposed to the atmosphere an example being clay which is formed by weathered feldspars. Kaolin Clay (white or porcelain clay is another common name) is the highest quality of the felspathic clays and is used to make high quality ceramic tile such as porcelain.

Mica: This is another mineral which can be various colours. The most pure is Muscovite (named because it was found near Moscow ) which is clear due to the inclusion of water.

Hornblende; this mineral contains primarily magnesium, iron, silica and alumina. It can be black brown or green in colour and is present in the crystal structure of a great percentage of igneous rocks.

When you mix these four minerals together in different concentrations as well as adding other minor impurities it is easy to see why granite is formed in so many different colours. However the true geological definition of granite (“real” granite so to speak) is “any plutonic rock in which the mineral quartz makes up 10 to 50% of the felsic components and the ratio of alkaline to total feldspar is between 65 and 95%” The group of rocks defined by this definition is quite small so it is obvious that the much larger group using the granite tag does not fit absolutely into this definition. Does this mean that they are inferior? In my opinion the answer in the main is no. (There are certainly cases where some stone types are passed off as granite – a good example being bluestone or basalt – based on aesthetics alone. Some basalt, certainly in polished form can look exactly like granite but we are not discussing these cases, we are looking at stones that are sold as granite and have the same general and physical characteristics of granite). The majority of stone in the much larger commercial group of granite simply does not fit this definition because they fall outside these percentage ratios. However their actual characteristics and performance is almost the same due to the fact that they are still made of the same minerals and formed by the same natural processes.

Granite (both groups) is the oldest commercially used rock. The earth is considered to be at least 4.5 billion year old and began as a hot sphere of molten material. As this hot material or magma cooled crystals formed creating the earth’s surface which was covered by a layer of what we refer to as igneous rock. Granite is an example of this which means that many of the granites were formed one billion years before dinosaurs roamed the earth.

The texture, density, and to a large extent the porosity and stability of granite is determined by its crystal structure which is in turn determined by the rate of cooling. Many of the people I meet in our industry think that the darker the colour of the granite the older the stone and the deeper under the earths surface it was created. The opposite is in fact the case. The deeper a granite is formed under the earths surface the slower the magma cools resulting in the formation of larger crystals as well as generally lighter colours. A good example is what is commonly called Kashmir although this stone can be called different names in different markets.

The closer the magma is to the surface the faster it cools forming a much smaller crystal lattice as well as generally darker colours such as Absolute Black.    

The significance of this is that the smaller the crystal the tighter and denser the complete crystal structure will be. Hence it is the darker coloured stones created nearer the earth’s surface that have the higher structural strength, the lowest porosity and the best overall stability. In contrast the lighter coloured granites are the ones that are the most porous, have lower overall structural strength and stability and are the oldest. It must be pointed out that the overall structural strength, stability and porosity is still very good for all granites when compared to other types of stone it is simply a difference between granites that I am referring to.

The importance of understanding the minerals and process that forms granite is that so called real granite can vary in its characteristics. It can be light or dark in colour and have a small, dense, tight crystal structure or a large crystal which can create more instability, porosity etc, influencing how the granite will perform. This is also true of the “other” granites and is the key to the original questions posed at the start. There is no doubt that only a small percentage of granite sold is “real” or geological granite, the remainder being termed granite but in fact being a large array of different geologically defined stones such as larvikite, granidorite, , gabbro, anorthiste etc. However to labour this fact is I think to miss the point. In general the commercial term granite is used to describe stones that exhibit  particular characteristics, namely stones that are hard, durable, low in porosity, take a high polish and perform well in harsh environments to name a few. Real and commercial granite fit this description hence I feel in the main the difference between the two groups based on the geological definition is academic. The more important issue to highlight is how the granite is formed and the type of crystal structure it has as this has a direct bearing on how the granite will perform. Selecting a lighter coloured granite for example in most cases will mean having higher porosity hence the need for a sealer where as a darker younger granite being much denser may not need one. In conclusion the answer to the original question is really that there is no significant difference in performance between the “real” and “commercial” granites and therefore questioning whether you have purchased real granite or not is simply not important.     

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