What is Quartz?
Quartz is the commonest of minerals. It is simply, made up of silicon and oxygen which are the two most abundant elements in the Earth's crust, and when combined, are commonly just called silica.
The two types of 'crystalline' quartz
There are two main types of 'crystalline' quartz; macrocrystalline quartz which occurs as individual, sometimes very large crystals, and sometimes in geodes, such as rock-crystal (colourless), amethyst (purple) and citrine (yellow-brown).
The other is polycrystalline quartz, which may also be called microcrystalline or cryptocrystalline quartz depending on the size of the grains, which doesn't have any external regular shape, but is made up of compact masses of minute crystals. Examples include chalcedony, and its varieties agate and carnelian.