Quartz is a chemical compound of silicon and oxygen the two most abundant elements in Earth's crust. It can be found in every geological environment and forms at all temperatures.
In their purest form quartz crystals are colourless and transparent. The presence of impurities causes a wide variety of colours. Impurities can also cause a crystal to become slightly cloudy or virtually opaque with no significant change in colour.
The greyish-white colour and/or cloudy appearance of milky quartz is caused by microscopic impurities of fluids. These became trapped in the crystal during the early stages of its formation.