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LuxeSci Show Notes: S1E10: Amethyst

Hello again.  Welcome back to LuxeSci - a podcast to re-ignite your wonder by exploring the science behind luxury items.  I’m Dr Lex, PhD, infectious disease expert and February birthday.  I’m joined by my co-host Dimos, or should I say Dr. Dimos, also a PhD, electrical engineer and also a February birthday. This is episode 10 so we’re changing things up a little bit in terms of format and content.  Dimos and I will be moving towards a more traditional co-hosting format where we both present on the science of the luxury item we’re discussing (yay more Dimos!).  Second, we are starting the first in a series of themes that we will be discussing.  That means if you like the theme, you can expect a series of episodes on that theme.  If not, I guess tune in when the current theme is done.  The first theme is jewelry so we’ll be exploring all the beautiful and luxurious materials used to manufacture jewelry.  We missed the boat for discussing the February birthstone in February but since both our birthdays are in February, we decided we have the prerogative to turn back podcasting time and cover…Amethyst.  Now, I’ve been gifted some beautiful amethyst jewelry and geodes over the years.  Dimos - any amethyst in your life?


Dimos - any stories about amethyst?


We hope you like this new format and so without further ado, here’s a microbiologist and electrical engineer’s take on amethyst


Background (Alexis)

  • Amethyst is a violet colored quartz crystal 

  • Quartz = hard, crystalline mineral made of silica dioxide (we’ve heard of silica in our last episode on garnet)

  • Second most abundant mineral on earth’s continental crust

  • Amethyst occurs in light lavender to deep purple

  • color is due to irradiation, impurities of iron and in some cases transition metals and the presence of other trace elements in the complex crystal lattice structure

  • Amethyst has a hexagonal crystal structure and rates a 7 on the Mohs Hardness scale

  • The highest quality amethyst is found in Siberia, Sri Lanka, Brazil and Uruguay and other areas in East Asia

  • The Greeks believed that amethyst gems could prevent intoxication 

  • Amethystos - not drunken

  • Wine goblets were carved from it

  • It was used as gemstone by the ancient Egyptians

  • Medieval European soldiers wore amethyst amulets as protection in battle

  • Believed the stone could heal people and keep them level-headed

  • The meaning of amethyst in cultures has varied over time and by culture

  • It was as expensive as rubies until the 19th century.  It was, until then, believed to be very rare but then huge deposits of amethyst were found in Brazil, downgrading it to being a semiprecious stone.

  • For individuals’ who are into crystal healing - amethyst is supposed to have the properties of calmness, balance and peace and people use it to eliminate impatience

  • Interesting fact: the particular website I found this on said there was no issue with using synthetic amethyst instead of natural amethyst.  I wonder if the authors would feel the same about other synthetic items.


Science

  • As we just mentioned, amethyst is a type of quartz crystal.  Quartz is usually clear except when it contains impurities like, iron, aluminum or titanium. 

  • Fun fact - a study from 1925 in the journal American Mineralogist first postulated that the purple of amethyst is due to iron and smoky quartz gets its color from silicon

  • But how does this color arise - a paper published in American Mineralogist in 1985 by Alvin Cohen (no collaborators I guess) showed that iron impurities at a specific spot in the quartz crystal (terminal major rhombohedral faces) protects against a smoky color developing if there’s enough iron.  These interfaces of the crystal and the iron (or other impurity) are called color centers. Both metals are present in the stone as ions (charged particles). Specifically, if the amount of iron outweighs the amount of aluminum in the crystal

  • It’s not enough, however, for the iron to just be present in the crystal.  The iron undergoes oxidation (remember that one, oxidation is the loss of electrons) so that instead of having a +3 charge, it has a +4 charge. 

  • This extra electron further blocks the aluminum ions preventing any smokiness to the color

  • The actual purple comes from the absorption band related to the +4 Fe

  • A little about color and absorption bands: absorption bands are “ranges of wavelengths, frequencies or energies in the electromagnetic spectrum which are characteristic of a particular transition from initial to final state”

  • So in this case, there’s an absorption band for when iron loses an electron and goes from Fe+3 to Fe+4.  

  • This changes the wavelengths of light that are absorbed by the crystal.

  • Absorbed wavelengths means that the crystal will appear the complementary color

  • For example, yellow light with a wavelength of around 570nm being absorbed would give the crystal a purple color = amethyst!!

  • So in summary = iron ions in the quartz crystal lose an electron to protect against smoky coloring caused by aluminum ions and the resulting iron ions yield a purple color to the stone

  • One recent study published in 2020 that I thought was fascinating heat treated amethyst to see what would happen to that purple color

  • The study was conducted by Renping Cheng and Ying Guo and was published in the journal Scientific Reports

  • The scientists heated amethyst under different conditions and measured not only what happens to the color but also what happens to the crystal structure

  • They found that:

  • Above 600C the amethyst is milky and turns a lighter purple, likely due to the destruction of the crystal shape and the color centers imbedded in it

  • They verified previous research that showed an absorption band at 545nm being key to the purple color and related to the charge-transfer of Iron and oxygen (Fe3+ to Fe4+)

  • Up until 420C of heat treatment, the purple color gradually fades with higher temperature

  • Between 420 and 440 is what the authors called the prasiolite phase.  Prasiolite is a green quartz

  • Above 440C, the amethyst because citrine (which is a yellow-ish quartz stone), with optimal citrine-making temperature of 560C

  • Above 600C = milky amethyst

  • 580C and above = irreversible color transition for the stone.  It won’t go back to purple if you cool it down

  • Interestingly - they found that that darker the initial purple color, the easier it was to change the color of amethyst as it was heating.

  • Fun Science

  • Now some fun science related to the color amethyst and not the actual stone

  • Remember how amethyst was considered to be protective against intoxication by the ancient Greeks?

  • The Amethyst Initiative is trying to get chancellors and presidents of universities around the country to call on elected officials to lower the 21 and up drinking age here in the US with the argument that it doesn’t prevent college kids from getting alcohol and may lead to a culture of dangerous binge drinking.

  • A team of researchers published a paper in the journal Alcohol Clinical and Experimental Research in 2012 forecasting the impact of lowering the minimum legal drinking age

  • They found that lowering that age may not result in less heavy episodic drinking due to an increase in campus wetness (how much alcohol is on campus)

  • Since the drinking age is still 21 in the US I guess this initiative may have had to rethink their strategy

  • Finally - a paper published in Applied Optics in 2020 documented a very rare amethyst moon in the region just south of the Tropic of Cancer during a lunar eclipse on January 20, 2019. 

  • Fun fact - the tropic of cancer is the most northerly circle of latitude on earth where the Sun can be directly overhead.

  • In North America, it hits in Mexico though it drifts because of a slight wobble in earth’s longitudinal alignment. (though this is a very slow wobble, varying about 2.4 degrees over 41,000 years)

  • The amethyst moon was seen by Leon, Mexico where the color transitioned from brownish red to amethyst blue (mixture of dark blue and some red)

  • Glossary

  • Oxidation - loss of an electron by a substance

  • Ion - charged particle

  • Prasiolite - green quartz


  • Fun cocktail party facts quiz

  1. What was amethyst associated with in Ancient Greece?  (protection from intoxication)

  2. How does amethyst get it’s purple color - iron impurities in the crystal

  3. How can you make citrine from amethyst - heat it to above 440C





I hope you’ve enjoyed exploring amethyst with me and Dimos and I hope you remember a little fact from this episode the next time you see amethyst.  Thanks for listening to this episode of LuxeSci.  A very special thank you to my audio engineer and co-host, Dimos.  Our theme music is Harlequin Moon by Burdy. We’re on Twitter and Instagram at luxescipod and our website is luxesci.podcastpage.io.  As always, please subscribe and review and a special request this week, if you like us, follow us on Twitter where we’re posting our weekly little luxuries on Mondays, these are posted dedicated to the little luxuries that get us through the start of our weeks.




References:


Fun science


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