By Yaĝé Enigmus
The colour of amethyst indisputably makes this gemstone one of the great treasures of the mineral realm and the most valuable single-colour variety of quartz. Residing among the most recognizable and most well known of all gems, this gorgeous purple stone can be easily identified by people of all ages and walks of life. Amethyst has long been important to human beings, and can be found in almost every country on Earth.
Amethyst is a variety of quartz characterized by its unique purple-violet colouration. This stone can display a multitude of different hues, with a range that includes pinkish-lavender, magenta, mauve, violet, purple, and indigo with the additional potential to exhibit faint tints of red and blue. Deep rich purples with a greater proportion of blue-indigo tones than red are considered ideal, with the most valuable colour being known as “Siberian purple” or “Russian purple” in reference to amethyst of this colour that was once mined in Russia. Lighter colours of amethyst are generally considered to be less valuable, but amethyst of a light lavender tone referred to as “rose de France” has recently become more popular for use in gemstone jewellery. Today notable sources of amethyst include Brazil, Uruguay, Bolivia, Mexico, Canada, Russia, Austria, Zambia, and India.
Quartz often acquires colour during formation when metals and other elements get trapped inside the growing crystal. The colour of amethyst primarily results from impurities of Fe3+ ions within the quartz structure that have been exposed to gamma radiation and during formation at certain temperatures. The radiation exposure causes the iron ions to lose an additional electron and become Fe4+ ions, which then combine with oxygen to form an FeO4 colour centre. This colour centre is responsible for the violet base colour seen in amethyst, but small amounts of other elements contribute to the gemstone’s hue. It is the combination of FeO4 and other trace elements that truly brings out the colour of amethyst, as only a small amount of iron is required to produce the desired hues when the correct mixture of other elements is present. These other elements are responsible for the wide range of colours that amethyst may display and the variation in hue that is seen in amethyst crystals from different localities, as the local geology would cause different combinations of trace elements to join the iron impurities within growing quartz. This is why one may see a striking difference in hue and tone when comparing amethyst from two different locations, such as the light magenta amethyst often found in Brazil and the deep indigo-violet amethyst found in Uruguay. Such trends in the colouration of amethyst from different localities can sometimes be used to help narrow down where amethyst may have come from if its origin is unknown or high level laboratory test is unavailable.
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Because of the chemistry involved, the colour of amethyst is not stable under certain conditions. Exposure to UV radiation for extended periods of time unfortunately causes a destabilization of the FeO4 colour centres and results in a loss of colour. For this reason amethyst crystals should not be left in direct sunlight for long periods, and gemstone jewellery made with amethyst should not be worn when taking part in activities that involve spending large amounts of time in the sun.
It is possible to treat amethyst because of the chemical basis for its colour. When heating amethyst to 300-400°C, partial destabilization of the colour centres may cause the stone to turn a golden colour much like natural citrine, or in some cases the stone may turn a greenish colour resembling natural prasiolite depending on the trace element content of the amethyst. When heated, the colour centres can also fully destabilize, causing the stone to lose colour and become lighter, a phenomenon that in rare circumstances may be taken advantage of in order to lighten the colour of amethyst that is too dark. Additionally, radiation can be used when treating amethyst to help induce brighter greens or golden colours than would typically result from using heat alone. Because of the role that radiation plays in the formation of amethyst crystals, it is possible to also use radiation treatments to darken light crystals, or re-induce purple and violet hues into stones that have lost their colour.
A classic burst of colour, amethyst is sure to dazzle and impress both those who love gemstones and those who know very little about them. When designing bespoke jewellery, amethyst makes a wonderful choice for both center stones and accents, as this gem is both hard and affordable. With such distinct colour and rich history, amethyst is an absolute must have in every jewellery and gemstone collection.
© 2020 Yaĝé Enigmus a.k.a. Kevin Back