Amethyst is a variety of quartz, a common mineral composed of silicon dioxide. This mineral is primarily recognized for its appealing purple coloration and is widely used in the jewelry trade and for decorative mineral specimens. Though once considered a precious stone alongside diamonds and rubies, the discovery of large deposits has made it more widely accessible.
The Signature Color and Hue
The most defining feature of this gemstone is its spectrum of purple, which can range from faint pinkish-lilac to a rich, deep violet. The lightest shade is often marketed as “Rose de France,” a pale lavender hue. Conversely, the deepest, most saturated purple is historically referred to as “Siberian quality.”
The purple color is not an inherent part of the quartz structure but is caused by specific impurities and geological processes. Trace amounts of iron are incorporated into the crystal lattice, substituting for silicon atoms during formation. This iron requires exposure to natural gamma radiation from the surrounding host rock to activate the color centers.
This combination of iron impurities and natural radiation results in the selective absorption of light wavelengths, allowing the purple light to pass through. Amethyst frequently displays color zoning, where the purple is unevenly distributed within the crystal. Often, the most intense color is concentrated near the tips or terminations of the crystals.
Physical Structure and Form
Amethyst is a member of the trigonal crystal system, forming crystals that typically feature a six-sided prism capped by a six-sided pyramid. It primarily crystallizes from silica-rich hydrothermal fluids that cool within open spaces in the earth’s crust. One of the most recognizable forms of amethyst is the geode, a hollow, rounded rock cavity lined with thousands of inward-pointing crystals.
These crystals often form in massive, raw clusters known as drusy, where a carpet of small, terminated crystals covers a matrix rock. Polishing and tumbling the material transforms the rough exterior into a smooth, glossy surface.
When a piece of amethyst is faceted into a gem, its appearance is maximized by exploiting its transparency and color depth. The characteristic crystal habit defines its raw form. The overall physical presentation can therefore vary widely, from a rough geode half to a meticulously cut, eye-clean stone.
Key Traits for Identification
One of the most practical traits for identifying amethyst is its relative hardness, a measure of its resistance to scratching. Amethyst registers a 7 on the Mohs scale of mineral hardness, a rating that makes it durable enough for all types of jewelry. This hardness is also a useful way to distinguish it from softer purple minerals or glass imitations.
The stone exhibits a vitreous, or glassy, luster when light reflects off its surface, especially when polished. Its transparency ranges from completely transparent in high-quality, gem-grade specimens to translucent. Like all quartz varieties, amethyst lacks cleavage, but instead displays a curved, shell-like conchoidal fracture when broken.