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"The stone is an amethyst; but I, the tipler Dionysus, say, 'Let it either persuade me to be sober, or let it learn to get drunk.'"
–Plato the Younger
I must start our foray into this month's birthstone with a confession: I didn't think amethyst would be exciting. I find it nice but kind of ho-hum. So I decided to go big and explore the origins of those enormous amethyst geodes suitable for making into bathtubs. However, along the way, I got diverted by some very interesting facets of amethysts, including some pretty neato human history. So in today's post, we're actually not going to talk much about geodes at all. Believe me when I say we'll get there soon, though!
Right. So, everyone knows amethyst is a purple variety of quartz, yeah? And we're used to it being abundant and fairly inexpensive, aside from those jaw-dropping giant geodes and some really fabulous art.
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So, it may shock you just a little bit to learn that gem-quality amethyst used to be considered just as valuable as rubies, celebrated in myth and legend. Rhea the Titan gave it to Dionysus, god of wine, to preserve his sanity from the vine. Royals and nobles sported it proudly. It even ended up in many a country's crown jewels. It remained an expensive stone, reserved only for the fortunate, right up until ginormous deposits were found in South America in the 1800s. Once those mines started producing, amethyst became available in copious quantities, and now you can buy some really nice specimens for the price of a gourmet coffee or few. Good news for those who want a bit of their birthstone for their own selves, isn't it?
Even without those sources, amethyst is fairly easy to track down, even if gem-quality specimens are less abundant. It's found most places where quartz ends up: in both extrusive and intrusive igneous rocks, in metamorphosed rocks (especially in alpine-type fissures), hydrothermal veins, rocks deposited by hot springs, and even some sedimentary rocks. You need just these basic ingredients for amethyst to form: pockets or cavities suitable for deposition, silica-enriched water around 50-250°C, a skosh of ferric iron (Fe³+), and gamma radiation.
Yup. Same radiation that creates your basic Incredible Hulks. Instead of the big green guy, they could've gone with purple and it would've been science.
See, you can have clear quartz that has iron substituting for some of the silica in its crystal lattice. It might have just as much iron as amethyst does, but without some nice radiation, it won't show off its amethystine potential. But don't worry – it doesn't take Hulk-sized doses of gamma rays to form our gorgeous purple gems. Just the natural bits will do, which is why amethyst is so much more abundant in igneous than sedimentary rocks. Igneous rocks like basalt have trace amounts of radioactive minerals like radium 226, thorium 232, potassium 40, and uranium 238. Studies have shown that the levels of radiation within these rocks is generally not enough to cause humans any health concerns, but it's quite enough to cause the Fe³+ atoms to lose an electron and create that dazzling purple.
That lovely purple, shading from the palest lilac to good deep violet and royal hues, is what gives amethyst its common name. Ancient Greeks thought it looked rather like wine, and from there flights of fancy took off and led them and the Romans to think that maybe the gem could be used to ward off drunkenness. So, they named it amethystos, the Koine Greek word for "not intoxicated." People would wear jewelry made from it, or drink from glasses carved from it. Chances are they got drunk regardless, but at least they did so in style.
If you look closely at amethyst crystals, you'll probably see that the tips are generally darker than the bases. We're pretty sure that's due to the iron mix in the water changing as the crystal slowly grows. If you have the right equipment, you could see that the colorless or pale bits of the crystal don't have much iron in, while the iron content increases as you get to the darker portions. There still won't be a lot of iron, though - maybe about ten to a hundred parts per million.
The base of the amethyst crystal given to me by my dear friends Ziggy and Dali shows the shading from clear to purple caused by variations in iron content. Credit: Dana Hunter
That little bit of iron does some pretty wild things to the color depending on what sort of radiation it's exposed to, and in what amounts. We've seen what gamma radiation does to it, but plain old UV light will also have an effect. If you leave your amethyst in sunlight or under other UV sources for too long, its color will fade. And if you expose amethyst to heat, you'll see the color fade as well. Sometimes, instead of gray or clear crystal, you'll end up with vivid yellows that look a lot like citrine. The neatest bit of this chameleon exercise, though, is if you can safely expose your amethyst to X-rays, you can return it to its original purple hues. That is a pretty nifty trick!
That covers the basics of these seemingly ordinary but surprisingly extraordinary crystals. Next, we'll visit an ancient Egyptian amethyst quarry that supplied generations of Egyptians and Romans with gorgeous purple gems, and then we'll travel to South America to learn how those extra huge geodes happen.
References:
Al-Zahrani, Jamilah (2017): Gamma Radiation Measurements of Naturally Occurring Radioactive in Igneous Rocks and Its Radiological Complications. World Journal of Nuclear Science and Technology, vol. 7, issue 03, pp. 136-144
Dennen, William H. and Puckett, Anita M. (1972): On the Chemistry and Color of Amethyst. Canadian Mineralologist VoL 11, pp. 448-456
Gemological Institute of America: Amethyst and Amethyst History and Lore (retrieved 2/28/2019)
Geology.com: Amethyst: The world's most popular purple gemstone (retrieved 2/28/2019)
Mindat.org: Amethyst (retrieved 2/28/2019)
Sententiae Antiquae: Amethyst, the Sober Stone (retrieved 2/28/2019)
The Quartz Page: Amethyst (retrieved 2/28/2019)
Voudouris, P. et al (2013): Amethyst occurrences in Tertiary volcanic rocks of Greece: mineralogical and genetic implications. Bulletin of the Geological Society of Greece, 47(1), 477-486.
Voudouris, P. et al (2018): Amethyst Occurrences in Tertiary Volcanic Rocks of Greece: Mineralogical, Fluid Inclusion and Oxygen Isotope Constraints on Their Genesis. Minerals 8, no. 8: 324.