Quartz (SiO2). Quartz is the most abundant single mineral on earth. It makes up 12% of the earth's crust, occurring as a variety of igneous, metamorphic and sedimentary rocks….~… Quartz is commonly separated into two groups depending on the size of the individual grains or crystals; macrocrystalline quartz in which individual crystals are distinguishable with the naked eye, and cryptocrystalline quartz in which the individual crystals are too small to be easily distinguishable under the light microscope. …~… Some of the macrocrystalline quartz varieties are: Amethyst, Ametrine, Cat's-eye Quartz, Citrine, Phantom Quartz, Rock Crystal, Quartz, Rutilated Quartz and Smoky Quartz. ..~…