Quartz is one of the most well-known minerals on earth. It occurs in basically all mineral environments, and is the important constituent of many rocks. Quartz is also the most varied of all minerals, occurring in all different forms, habits, and colors. There are more variety names given to Quartz than any other mineral. Although the Feldspars as a group are more prevalent then Quartz, as an individual mineral Quartz is the most common mineral. 

 

Most mineral reference guides list Chalcedony as an individual mineral, but in reality it is a variety of Quartz. It is the microcrystalline form of Quartz, forming only occurs in microscopic, compacted crystals. This page deals only with the crystalline forms of Quartz. Chalcedony is listed on its own dedicated page in this guide. Other important varieties of Quartz, such as Amethyst, Citrine, and Agate, also have dedicated pages due to their popularity and varieties. 

Some forms of Quartz, especially the gemstone forms, have their color enhanced. Almost all forms of the yellow-brown variety Citrine are in fact heat treated. Much Amethyst is also heat treated to intensify color, and a green transparent form known as "Green Amethyst" or "Prasiolite" is formed by heat treating certain types of Amethyst. There is also a transparent sky blue form of Quartz crystals, as well as a wildly iridescent type that are synthetically colored by irradiation of gold. In some localities, Hematite forms a thin red or brown layer internally in the Quartz crystal, giving it a natural bright red to brown coloring, and sometimes even a mild natural iridescence. 

Quartz frequently forms the inner lining of geodes. Most geodes have an inner layer of larger crystalline Quartz, and an outer layer of Chalcedony or banded Agate.

 

 

Crystal Forms and Aggregates

Crystals, which are hexagonal in shape, vary in shape and size. Quartz crystals are unique and very identifiable with their pointed and often uneven terminations. Crystals can be in enormous prismatic and stubby crystals, or in pointed aggregates of such crystals. Crystals are usually striated horizontally, and are sometimes doubly terminated. Quartz crystal habits include drusy, grainy, bladed, as linings of geodes, as rounded waterworn pebbles, radiating, as pointy pyramids on a matrix, as dense agglomerations of small crystals, massive, globular, stalactitic, crusty, in nodules, and in amygdules. 

Crystals frequently twin; a famous twinning habit is the Japanese twin, where two crystals contact at a 90º angle. Quartz crystals may also contain a scepter growth, where the top of a crystal bulges out from the rest of the crystal, and may also form as phantom growth, where one crystal forms over another, leaving a ghosted form inside. 

The crystal structure of Quartz is a very complicated. As a result of a changeover from alpha to beta Quartz, crystals form as hexagonal prisms with modified crystal faces.