Member of the quartz family.
SiO4 is the base formula with an influx of Fe3+ and Fe4+.
Amethyst owes its color to high energy radiation, e.g. gamma rays from radioactive sources and the presence of iron built into its crystal lattice. The irradiation causes iron Fe(+3) atoms which probably replace the silicon in the center of the SiO4 tetrahedra to lose another electron and form Fe(+4), an unusual oxidation state of iron. Note that the colorizing iron and its precursor is probably not present in the lattice as ideal ions (Fe3+ and Fe4+), but in part bound covalently to oxygen to form a FeO4 group. The electron released from the iron by irradiation is probably taken up by another ion, but there are different models on how exactly the Fe(+4) color center is stabilized. According to a theory of Lehmann et. al., 1973, the electron released from the Fe3+ to form Fe4+ is captured by another Fe3+ elsewhere in the lattice which gets reduced to a Fe2+, so the color center would be a pair of [FeO4]2+/Fe2+ formed in this reaction:
[FeO4]– + Fe3+ → [FeO4]0 + Fe2+
However, other mechanisms including the role of other ions in the charge compensation have been proposed and the details of the nature and formation of amethyst color centers are still a matter of debate.
Do not expose an amethyst to direct sunlight for a long time. Very likely it will pale out by the ultraviolet radiation. Some amethyst pales out really quickly and some very slowly, but you can’t tell in advance. Even the intense sunlight behind the window in your living room might do some damage, although the ultraviolet components are mostly filtered out by the window glass. This is actually true for all quartz varieties whose color is irradiation-induced (smoky quartz, citrine, pink quartz). If exposed to the direct sunlight, it is recommended covering them with a black cloth so as to block the UV radiation.
As the color of amethyst is caused by iron-based color centers, iron minerals are often companions of amethyst. Of those two, goethite (FeOOH) and hematite (Fe2O3) frequently occur as inclusions.
Amethyst is the violet-colored sister stone of golden citrine quartz. Color-zone ametrine is the bicolored combination of both amethyst and citrine. The only difference between amethyst and citrine is the level of iron impurities in amethyst. When amethyst is heated at high temperatures of around 470ºC to 750ºC, iron impurities are reduced and can turn violet amethyst into golden ‘heated’ citrine. Lower grade amethyst stones are often heated to produce golden colored citrine stones. Heated citrine will typically appear more reddish when compared to unheated citrine.
Since purple is considered to be one of the royal colors, amethyst has a historical importance as an insignia of power. Fine amethysts are featured in the British Crown Jewels and they were also a favorite of Catherine the Great and Egyptian royalty. Amethyst also holds a high place in the ranks of the Christian church and was referred to as “the stone of bishops”. The Greek word “amethystos” translates into “not drunken” and it was often worn as an amulet to protect against intoxication. Since amethyst was considered an antidote against drunkenness, many wine goblets were carved from amethyst stone. Still to this day, violet-purple amethyst is a symbol of sobriety.
Amethyst has been highly esteemed throughout the ages for its stunning beauty and powers to stimulate, and soothe, the mind and emotions. It is a semi-precious stone in today’s classifications, but to the ancients it as a “gem of fire,” a precious stone worth, at times in history, as much as a diamond. It has always been associated with February, the month the Romans dedicated to Neptune, their water-god, and is the traditional birthstone of that month. It is the stone of St. Valentine and faithful love, and signifies ecclesiastical dignity as the Bishop’s Stone. It carries the energy of fire and passion, creativity and spirituality, yet bears the logic of temperance and sobriety. Many people keep it and wear it for healing and well being.
Amethyst is found on virtually every continent on the planet. Some of the best and largest crystals come southern Brazil and Uruguay.
By JJ Harrison (email@example.com) (Own work) [GFDL 1.2 (http://www.gnu.org/licenses/old-licenses/fdl-1.2.html) or CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Amethyst. Magaliesburg, South Africa
Humanfeather at English Wikipedia [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
Facet Cut Amethyst
By Pithecanthropus4152 (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Emerald cut amethyst
© Marie-Lan Nguyen / Wikimedia Commons, via Wikimedia Commons
Portrait of Roman Emperor Caracalla; inscription in Greek letters and cross added at the Byzantine period in order to transform the portrait in that of St. Peter. Amethyst intaglio, ca. 212 CE. From the treasury of the Sainte-Chapelle in Paris.
Amethyst crystal geode from outside and inside / de: Amethyst Kristalldruse.
German Wikipedia, original upload 15. Dez 2003 by Saibling