Quartz is a mineral that appears in many forms and colors. Fundamentally, quartz is a single chemical compound, silicon dioxide (SiO2), structured as a continuous framework of silicon-oxygen tetrahedra. It is one of the most common minerals found on Earth, constituting about 12% by mass of the lithosphere. The numerous names used to describe quartz are not different minerals, but varieties defined by differences in crystal size, color, and the presence of trace elements or inclusions.
Defining the Major Structural Groups
The scientific classification of quartz varieties simplifies the numerous names into two primary groups based on the size of the individual crystals. This structural difference dictates the outward appearance, ranging from transparent gems to opaque, dense masses. The first group is macro-crystalline quartz, where individual crystals are large enough to be seen with the naked eye. The second major category is cryptocrystalline or microcrystalline quartz, which has crystals so tiny they can only be resolved using a microscope.
Varieties with Visible Crystals
The macro-crystalline group encompasses the varieties most people associate with classic quartz crystals, defined primarily by their color and clarity. The purest form is Rock Crystal, which is colorless and transparent, often forming six-sided prisms. Its clarity comes from the virtual absence of impurities in the SiO2 structure.
The purple variety, Amethyst, owes its hue to trace iron impurities combined with natural irradiation. This process creates color centers within the crystal lattice, resulting in the characteristic violet color. Citrine is the yellow to orange-brown variety, with its color also stemming from ferric impurities. Natural citrine is quite rare, and much of the commercially available material is heat-treated amethyst.
Smoky Quartz ranges from light brown to nearly black, resulting from natural gamma radiation acting on trace aluminum within the crystal structure. This creates a color center that causes the darkening. Rose Quartz is the delicate pink to reddish-pink variety, often occurring in massive form rather than distinct crystals. Its coloration is attributed to minute, fibrous inclusions or trace amounts of titanium, iron, and manganese.
Varieties with Microscopic Crystals
The cryptocrystalline group includes varieties collectively known as Chalcedony, which are characterized by their dense aggregates of microscopic quartz crystals. These varieties form when silica-rich water deposits the material in cavities, often resulting in a translucent to opaque appearance with a waxy feel. The specific name given to a chalcedony specimen is determined by its color, pattern, and opacity rather than its fundamental crystal structure.
Agate is a prominent example, distinguished by its concentric layers or bands of varying colors and translucency. These bands form through the rhythmic deposition of silica layers within a cavity. A related variety is Onyx, a form of agate that exhibits straight, parallel bands, most commonly seen in black and white layers.
The opaque, granular varieties of chalcedony include Jasper, typically colored red, yellow, brown, or green due to the high inclusion of foreign materials like iron oxides. Another specific type is Bloodstone (heliotrope), characterized by its dark green body color with distinct red spots. The green color is due to chlorite inclusions, while the red spots come from hematite.