I often get asked, “What is granite”?

A common answer is that it is natural rock created when the earth was cooling. However, I want to provide some more detailed information here and lead you to an understanding of why granites are not all are created equal.

Granite is an igneous rock formed by crystallization of magma (molten rock below the surface). Once magma reaches the surface it is called Lava so granite was never lava. Granite’s origin is ‘intrusive’, meaning that it was formed below the surface. It is actually formed at a considerable depth in the Earth’s crust.

Today, many industry professional believe that a natural rock is a granite if it has over 65% quartz mineral content. I admit that I have even stated this when I felt a generalization was needed about the hardness of granite. However, it is inaccurate because granite is in fact at least 65% silica and only a minimum of 20% quartz. But isn’t quartz and silica the same thing? Let me explain how they are different with this analogy.

52 Card pickup:

Do you remember this card game where you hold a nicely stacked deck of cards in your hand and then flick the whole deck up in the air! Well, that portrays the difference between quartz and silica. Silica is a random scattered batch (amorphous) of the organized quartz mineral (crystalline structure). The nicely stacked deck of cards is quartz, and the group of random cards all over the table is silica. Is silica the same as quartz? Is quartz the same as silica? Well, they both have the same proportion of elements, but their structure is not the same – and this is very important to understanding the differentiation of a specific granite’s quality for a specific use.

Let’s now review the other minerals in granite.

These other common minerals are what usually classify the general color of the granite. For example, pink and white granites tend to have more pinkish orthoclase and microcline over plagioclase. White granites tend to have more albitic plagioclase, yet have some dark mica and hornblende too. Basically, these other minerals are different colors and their combination along with the color and size of quartz and silica make up the designation (and general color) of the granite.

Another analogy – Making Homemade Ice Cream:

We have a hand crank wooden ice cream maker. Every summer, I am called by my children to crank up some nice homemade ice cream. One day it occurred to me that making ice cream like making granite. Before you crank that cream (the silica), open the top, and add some cherries, blueberries, chocolate chips, and crushed cookies (all these are like the feldspar, mica and other minerals). Now wait! Don’t crank yet because it you do, it will not be a like a granite. You need to add at least 20% nuts (quartz). The ice cream goes through a hardening process similar to crystallization and in far less time than it took the earth to cool and form granite – you have some nice firm ice cream!

In summary and what it means to you next project:

If granite was actually 65% quartz, as most think, then there would be fewer bonds (micro seams) per area between minerals; thus fewer weak points for stress and absorption. Because silica, in its random structure, is the primary component of granite, you have a very hard rock but the bonding of the silica around the other elements and to itself will vary by overall natural occurrences during its formation (time, heat and pressure). Some granite selections will be more absorbent than others; therefore, always seal granite. Some granite selections will be more fragile; therefore, consider its specific use and hire a professional to fabricate and install granite. Most problems occur because the end user’s expectations are not met. Be knowledge and you will be happy in the end.

My Best,
Tom Cordova