Granite, Marble, Soapstone Countertops - Buyers Guide
What is Granite
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. Rock that cools slowly has larger more dramatic crystals but is often weaker. There is a lot of stone on the market today that is sold as granite but is, in fact, not true granite. 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 if highly stressed 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. Most granites are affected very little by common food acids.
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. Some minerals will take on a higher polish than others, so there is always some variability. 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 quarry and processing factory before final polishing. This helps to strengthen weaker stones and fills some of the natural pits in the surface. The color of some "resined" slabs will age or change over time, typically darkening, and on rare occasions the resin will react with sealer applied after installation to produce a milky finish. This is a hot topic in the industry at the moment and some manufacturers do not provide full information about their manufacturing processes.
For some interesting pictures of stone quarrying and manufacturing operations in Brazil click here