Quartz is the most common mineral on the face of the Earth. It is found in nearly every geological environment and is at least a component of almost every rock type. It frequently is the primary mineral, >98%. It is also the most varied in terms of varieties, colors and forms. This variety comes about because of the abundance and widespread distribution of quartz. A collector could easily have hundreds of quartz specimens and not have two that are the same due to the many broad categories. The specimens could be separated by answers to the following questions: color?, shade?, pyramidal?, prismatic?, druzy?, twinned?, sceptered?, phantomed?, included?, tapered?, coated?, microcrystalline?, stalactitic?, concretionary?, geoidal?, banded?, etc. Multiple combinations of these could produce hundreds of unique possibilities.
Some macrocrystalline (large crystal) varieties are well known and popular as ornamental stone and as gemstones.
Cryptocrystalline (crystals too small to be seen even by a microscope) varieties are also used as semi-precious stones and for ornamental purposes. These varieties are divided more by character than by color.
The primary varieties of chalcedony are as follows:
Quartz is not the only mineral composed of SiO2. There are no less than eight other known structures that are composed of SiO2. These other substances and quartz are polymorphs of silicon dioxide and belong to an informal group called the Quartz Group or Silica Group. All members of this group, except quartz, are uncommon to extemely rare on the surface of the earth and are stable only under high temperatures and high pressures or both. These minerals have their own unique structures although they share the same chemistry, hence the term polymorph, which means many forms.
Quartz has a unique structure. Actually, there is another mineral that shares quartz's structure, and it is not even a silicate. It is a rare phosphate named berlinite, AlPO4, that is isostructural with quartz. The structure of quartz involves corkscrewing (helix) chains of silicon tetrahedrons. The corkscrew takes four tetrahedrons in order to repeat itself, or three turns. Each tetrahedron is essentially rotated 120 degrees. The chains are aligned along the C axis of the crystal and interconnected to two other chains at each tetrahedron making quartz a true tectosilicate. This structure is not like the structure of the chain silicates or inosilicates whose silicate tetrahedronal chains are not directly connected to each other. The structure of quartz helps explain many of its physical attributes.
For one, the helix makes three turns and this helps produce the trigonal symmetry of quartz. Likewise a helix or corkscrew lacks mirror planes of symmetry as does quartz. The corkscrew structure would also disrupt any cleavage which requires a plane of weakness not found in quartz and breakage would result in the curved fracture, conchoidal, that is found in quartz. Quartz can also have left and right handed crystals just as a corkscrew can screw in a left handed way or in a right handed way. There are even some very difficult to identify crystals of quartz that are twinned with alternating one sixths of the crystal being right handed and then left handed.
Quartz is a fun mineral to collect. Its abundance on the Earth's surface is incredible and produces some wonderful varieties that don't even look like the same mineral. A collector must always be up on the many varieties of quartz and it sometimes embarrasses a collector to have collected too many specimens of such a common mineral. But nearly all collectors concede that you can never really have enough quartz specimens. Note that quartz is nearly a defining component of most geodes. The agate form typically lines the original cavity, creating a durable shell to contain subsequent mineral growth, which itself is often quartz crystals of one or more varieties. Geodes from Brazil are a good example, as layers of agate comprise the shell, lined with milky and/or colorless quartz crystals, topped with amethyst.