What is Quartzite?

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Whilst most rocks on the planet are made up of several different minerals some are made up of just one. Quartzite which is composed almost entirely of quartz formed through the metamorphism (alteration in form) of sandstone which is a common sedimentary rock that contains minute grains of quartz.

In its purest form quartzite is usually white or grey although the presence of iron can cause various shades of pink or red.  Inclusions of chromium rich mica cause its colour to turn green and when used for decorative purposes, the stone becomes known as green aventurine. Other colours which may also be present will be caused by inclusions of different minerals.
 
An extremely hard stone that's resistant to chemical weathering, it can sometimes resemble marble but unlike marble cannot be scratched with a knife.  The reason why it's so tough and durable is because of the network of tightly interlocking grains of quartz which form its crystalline structure.