The Amethyst is the best known Quartz partially due to its distinctive purple colour, and plentiful supply. Today, most Amethyst comes from Brazil and Uruguay but other supplies can come from many other places such as Russia, Sri Lanka, India and Germany.
“The name refers to its alleged ability to protect its owner from the negative effects of alcohol consumption, namely “methy”, the Greek expression for being drunk, and “a-methy-stos” would translate to “one that does not get drunk”. This association probably came up because of the color of red wine is sometimes similar to that of amethyst.” – http://www.quartzpage.de/amethyst.html
Amethyst forms in many different types of rock and forms, most amethysts share the same basic properties.
Firstly, the colour, which is down to high radiation levels, can range from pinkish-violet to dark blue-violet. Tips colour can be distributed unevenly causing gradients of hue and saturation – the most saturated colour is most often at the tips of the crystals.
This site explains the science behind the formation of the crystals. The below paragraph explains the chemical requirements;
“Amethyst owes its color to high energy radiation, e.g. gamma rays from radioactive sources (Berthelot, 1906) and the presence of iron built into its crystal lattice (Holden, 1925). 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 (Lehmann and Moore, 1966). 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. For a review of the different recent theories, see Rossman, 1994, early theories have been discussed by Holden, 1925.” – http://www.quartzpage.de/amethyst.html