|Most of the synthetic quartz is hydrothermally grown under pressure and at medium temperature. Pure silicon dioxide or crushed natural quartz can be used as a starting material.
For further information on the process and an animation see the Hydrothermal Growth Process page.
It is possible to grow quartz as citrine (yellow), amethyst (purple), ametrine (amethyst/citrine), blue quartz, and even green quartz. Smokey quartz is not grown directly, but can be created by irradiating grown crystals (synthetic amethyst is made in a similar way).
Blue quartz has no natural equivalent. I often use it to try out new cuts that are unfamiliar to me.
Synthetic green quartz as no natural equivalent although there are some green to yellow-green quartz colors that can be created by heating natural smokey quartz.
The synthetic citrine and amethyst are produced in several color variations that mimic that found in nature.