MineralMonday: Amethyst and Other Quartz Varieties

This week's mineral: Amethyst and other Quartz Varieties

Amethyst is a variety of quartz. It shows a beautiful purple colour.

However, the colour isn't evenly distributed. Usually, the color is most intense at the tips and less intense at the base.

Amethysts are often found in druses. From the outside druses look like rocks, but once cracked open, they sparkle and show mesmerizing crystals that grew inside a hollow volcanic rock due to concentration imbalances of circulating fluids. These fluids contain dissolved minerals which were precipitated in certain areas (due to saturation).

Over time, beautiful druses form, as the crystals have a lot of time and enough space to grow into its typical trigonal crystal shape.

You also might have seen amethyst as rounded and polished stones. These are called tumbled gemstones and don't show the beautiful natural shape anymore, since they were smoothed and polished on purpose. Tumbled stones are great to collect and they're also used in jewelry, crafting, or are sold as souvenirs. 

There exist many other varieties of quartz beside amethyst: rose quartz, milk quartz, smoky quartz, berg quartz, agate, jasper, chalcedony, citrine, and others.

Perhaps you've heard of these varieties before but up until now thought they were separate minerals. In fact, they have the same chemical composition (SiO₂), but differ in colour. 

Berg quartz is a mineral that you can almost see through, it is very translucent! Usually when talking about quartz, people talk about this regular variety. I've already talked about the regular quartz last week. Read back here.

Where does the different colour come from?

As said before, quartz is made up of two chemical elements: silicon and oxygen. However, it is only MOSTLY made up of those. 

When crystals solidify (crystalize), it is very rare that a perfect crystal is built (we call perfect crystals 'idiomorphic'). Usually, there are lattice defects. Lattice defects are irregularities on atomic or molecular level, interruptions of the regular, orderly pattern. Or in other words: the position of some atoms or molecules is wrong or empty.

Additional trace elements may substitute other atoms within the lattice and subsequent irradiation can then cause coloration, as color centers will appear and absorb specific wave lengths of electromagnetic light, which is determined by the atom's electron configuration.

This is the case with amethyst. 

Inside the amethyst lattice, there exist inclusions of iron atoms. Subsequently, gamma irradiation causes the iron atom to lose an electron (it oxidizes from Fe³⁺to Fe⁴⁺). The iron cation replaces the silicon atom inside the tetrahedra (quartz is contructed by numerous SiO₄ silicon tetrahedras), forming an FeO₄ color center that causes the absorption of light and hence the resulting color.

Where does the radiation come from that causes this effect in nature?

The required radiation is produced by radioactive decay of other elements, such as potassium (K) in granite. Other isotopes such as from the elements radium (Ra), thorium (Th) and uranium (U) are radioactive as well and occur naturally in (igneous) rocks. 

Inside the amethyst, despite its intense, somtimes opaque coloring, only very few silicon positions are occupied by iron atoms, only about 10-100 ppm (parts per million).

The amethyst color can be caused artificially by irradiating a regular quartz crystal with x-rays.

If treated with heat, the violet colour will turn into yellow, orange or light brown. They then resemble the quartz variety citrine (but it is NOT citrine!).

If treated with ultraviolett light (UV rays), the amethyst's color will fade. This means if you love the color of your amethyst, don't leave it in the sun for too long!

I've posted a video of an amethyst geode and tumbled amethyst on my instagram. Click here to view it (you don't have to have an account to see it!).

This is only one way of how minerals get their color. At a later time, I'll talk about other processes which lead to stunning mineral colors. Subscribe to my email list below to stay in the loop!

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About the author

Daniela is convinced that by gaining deep insights into planet Earth and travel destinations you’ll create meaningful, grounding and memorable life and travel experiences. She explains fundamental geological processes that form and shape landscapes and combines these insights with philosophical and philanthropical views in her online courses, articles, and newsletter. She holds two bachelor's degrees in geosciences (B.Sc.) and business administration with tourism (B.A.). She is the owner and founder of EarthyMe, EarthyUniversity and the Science of Travel blog and the Stories of Earth newsletter.

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Image Credit

JJ Harrison. Amethyst. Magaliesburg, South Africa. Licensed under GNU Free Documentation License, Version 1.2 only and e Creative Commons Attribution-Share Alike 3.0 Unported

Text Sources

Mindat.org: Amethyst (retrieved April 5th, 2020) 

Chemie.de: Amethyst (retrieved April 5th, 2020)

Scientific American: The Origin of Amethysts May Leave You Tingly (retrieved April 5th, 2020) 

Voudouris, P.; Melfos, V.; Mavrogonatos, C.; Tarantola, A.; Gӧtze, J.; Alfieris, D.; Maneta, V.; Psimis, I. Amethyst Occurrences in Tertiary Volcanic Rocks of Greece: Mineralogical, Fluid Inclusion and Oxygen Isotope Constraints on Their Genesis. Minerals 2018, 8, 324.

Markl, Gregor (2015): Minerale und Gesteine, Mineralogie - Petrologie - Geochemie. 3. Aufl. Springer Spektrum. Berlin Heidelberg 2015. S.161 ff.