Quartz and other minerals made of silicon dioxide are grouped in the class silicates. They are sometimes called the Quartz Group or Silica Group. Members of the group other than quartz are uncommon to extremely rare on the surface of the earth. Quartz also belongs to the subclass Tectosilicates. Quartz is also the most varied mineral.
Specimens vary according to color, shade, transparency, size of crystals and crystal form.
There are hundreds of unique quartz varieties. Quartz gems also include rutilated quartz, citrine, ametrine, onyx, and chrysoprase. Amethyst One of the most celebrated quartz gems is amethyst. Adopted as the official gemstone of South Carolina and Ontario in 1969 and 1975, it is merely purple quartz. Amethyst's long popularity among royalty is due partly to the status of purple as a royal color, along with amethyst's great beauty.