How Granite is Formed
True granite is an “igneous” rock formed when molten rock cools deep inside the earth. The size of the grains and crystals of the minerals that make up the rock are determined by the temperature, pressure and speed of cooling. Rock that cools rapidly is made up of small crystals and tends to be harder and stronger.
Types of Granite
Rock that cools slowly has larger more dramatic crystals but is often weaker. A rock consisting of equal proportions of orthoclase and plagioclase plus quartz may be considered a quartz monzonite. A granite containing both muscovite and biotite micas is called a binary granite. The average density is 2.75 g/cm3 with a range of 1.74 to 2.80. There is a lot of stone on the market today that is sold as granite but is, in fact, not true material used in granite worktops. Sedimentary rocks (rocks deposited as layers of sediments) consist of cemented grains of sand, shale and other minerals – slate is a good example.
Minerals in Granite
The key minerals in granite are quartz (very hard), feldspar, and softer minerals such as mica which add special reflectivity and drama. Even though it is very hard, granite can be scratched because of the presence of softer minerals, and it can be broken or chipped because of the weaknesses along grain boundaries. Similarly some minerals in natural stone can be attacked by acidic liquids – lemon juice, coke etc, some more so than others. Another fact often misunderstood by customers is that the glossy shine of natural stone isn't caused by something coating the surface, it is the actual stone polished to a mirror finish using diamond polishing tools.
Waxes and polishes just produce a water-resistant surface and add some reflectivity - like waxing your car. Some slabs are coated with a resin at the factory before final polishing. This helps to strengthen weaker stones and fills some of the natural pits in the surface. Granite is found in all continents around the world and is generally the foundation of many orogenic belts or mountain chains. Most often granite is the underlying rock upon which sedimentary and other continental rocks rest. Granite is found in batholiths or large magma plumes that rose into the continental rocks. But it can be seen in lots of other intrusive features such as dikes, sills and lacoliths. Learn how granite is quarried and processed here