The name “green amethyst” is a widely used trade term, but it is considered a misnomer. The correct gemological designation for the transparent, green variety of quartz is Prasiolite. Prasiolite is a silicate mineral belonging to the quartz family, composed of silicon dioxide (\(\text{SiO}_2\)). The stone is chemically identical to amethyst but displays a distinct, delicate, leek-green color. This hue is highly sought after for use in jewelry.
Prasiolite: Defining the Green Quartz
Prasiolite is classified as macrocrystalline quartz, sharing its silicon dioxide composition with varieties like amethyst and citrine. The distinction between these stones is based solely on their color, which is determined by the presence and oxidation state of trace impurities. Amethyst is defined as the purple variety of quartz, meaning a true amethyst cannot be green.
The accepted term for the green stone is Prasiolite, derived from the Greek words prason and lithos, translating to “leek-green stone.” The trade name “green amethyst” persists because it capitalizes on the public recognition of the purple stone.
As a quartz variety, this green gemstone exhibits considerable durability for use in everyday jewelry. It registers a hardness of 7 on the Mohs scale, meaning the stone is robust enough for various applications. Prasiolite is typically transparent to translucent and ranges in tone from a very light mint green to a deeper, more saturated sage color.
The Creation of Color: Natural vs. Treated
The green color in Prasiolite is linked to minute inclusions of iron ions within the quartz crystal lattice. This coloration mechanism is similar to that in amethyst, but the final hue depends on how the iron compounds are altered. The vast majority of commercially available Prasiolite is produced through a controlled color transformation process.
The most common method is heat treatment, where specific types of amethyst are heated in a laboratory environment. When amethyst containing particular iron compounds is heated between \(400^\circ C\) and \(500^\circ C\), the heat changes the iron oxidation state. This chemical reaction shifts the stone’s color from purple to a stable green or yellowish-green.
A less common technique involves irradiation of natural quartz. This process uses electron or gamma rays to alter the crystal structure, resulting in a pale green hue. This irradiation-induced color is often less stable than the heat-treated variety and can fade if exposed to temperatures above approximately \(150^\circ C\).
Naturally occurring Prasiolite is extremely rare, accounting for less than one percent of the market supply. Natural formation is believed to result from a geological process. This natural transformation occurs when amethyst deposits are exposed to a combination of natural irradiation and subsequent geothermal heating, such as from nearby lava flows or intrusions, over millions of years.
Major Sources and Geological Context
The primary source for the amethyst material specifically suited to be transformed into Prasiolite is the Montezuma mine in Minas Gerais, Brazil. This deposit yields quartz that possesses the unique chemical composition required to turn green when subjected to heat treatment. Most of the world’s commercial Prasiolite is ultimately sourced from this Brazilian material.
These deposits are found in hydrothermal quartz veins that penetrate quartzite host rocks in the Serra do Espinhaço region. The formation of amethyst and Prasiolite is tied to the movement of silica-rich, hot water solutions deep within the Earth’s crust. This geological environment is responsible for depositing the quartz crystals with the necessary trace iron impurities.
While the Montezuma mine dominates the commercial market, natural occurrences have been documented in other locations. The first recorded discovery was in Lower Silesia, Poland, in the early 19th century. Other sources of natural or heat-treatable material include deposits in Thunder Bay, Canada, and specific areas of the Western United States.