True granite slab 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. Rock that cools slowly has larger more dramatic crystals but is often weaker.
There is a lot of stone slabs on the market today that is sold as granite but is, in fact, not true granite slabs. Sedimentary rocks (rocks deposited as layers of sediments) consist of cemented grains of sand, shale and other minerals – slate is a good example. Limestone consists of the fossilized shells of sea creatures that lived millions of years ago. Metamorphic rocks are those which started as one kind of stone and changed under heat and pressure into another type of rock. Marble and Gneiss (“nice”) are examples.
Many rocks that are called granites are in fact Gneiss or a kind of layered granite. None of this really matters so long as you can find the stone with the color, texture and strength that you need.
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.
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