| Quartz is silicon
dioxide, SiO2, the second most common mineral in the crust of the earth (less
common than feldspar). Although found in abundance in nearly all rock types, gem quartz
nearly always comes from igneous rocks, principally pegmatites and cavities in volcanic
rock, or from hydrothermal veins. Quartz is the most diverse and abundant of all gem
materials, a fact reflected in its low price and popularity among lapidaries throughout
the ages. Amethyst, carnelian and other colored varieties (described below) were among the
first materials to be fashioned by early lapidaries for adornment and, like many other
gems, were once highly prized for presumed medicinal and talismanic attributes.
Gemologists recognize two broad categories of quartz, coarsely crystalline and finely (or crypto-, meaning hidden) crystalline. Coarsely crystalline varieties, like amethyst, citrine, smoky quartz and others (see below) occur as relatively large, single crystals that can be faceted or cut en cabochon. Cryptocrystalline quartz, called chalcedony (kal-sed'-nee), is composed of intergrown aggregates of microscopic or submicroscopic quartz crystals. Such materials, like agate, onyx, carnelian, and many others (see below) are commonly translucent to opaque (the many small crystals scatter light) and are thus usually cut en cabochon.
The principle industrial use of quartz is in the manufacture of electronic devices (radio and TV tuners, watches, radar, sonar, etc.) that use quartz resonators to generate waves of a fixed frequency, or to convert resonance at a given frequency to a current. These unique applications are possible because quartz has no center of symmetry and thus exhibits a property called piezoelectricity. Pressure applied to a quartz crystal generates a weak electric current. Alternatively, an applied alternating current causes a crystal to resonate with a fixed frequency. High purity, untwinned crystals are cut into small thin plates for these applications. Large synthetic quartz crystals, grown by a hydrothermal process, are produced in enormous quantities to satisfy the industrial demand. The same process is used to grow amethyst, citrine and other colors of coarsely crystalline quartz, some of which are unknown or rare in nature (e.g. blue quartz), for gem application. Of greatest concern is synthetic amethyst, which can be extremely difficult to reliably distinguish from its natural counterpart, but can often be detected by a lack of twinning (polariscope), its flawlessness (microscope), and weak to absent color zoning (immersion liquid). All of these characteristics are uncommon in natural amethyst.