Blogging a crystal structure a day in 2014


Contributed by

Helen Brand

February’s Birthstone – Amethyst

What does it look like?

This picture was drawn using Diamond structure visualisation software. Si is white, O is red

This picture was drawn using Diamond structure visualisation software. Si is white, O is red.

What is it?

It’s time for the second birthstone of the year, February's gemstone is Amethyst.

Hang on, that structure looks familiar – haven’t we had that already?!

Yes, sort of. Amethyst is a variant of Quartz and so has the same crystal structure. As we learnt on 14th January, Quartz comes in some different coloured variations. Amethyst is the purple variety.

The purple colour in amethyst has been widely attributed to several different impurities over the years but in fact the colour comes from Fe3+ impurities within the structure. More specifically, the colour comes from the irradiation of these Fe3+ impurities. In nature, this is probably due to gamma radiation produced in the decay of K40 in the nearby rocks – so if you want good colour to your amethysts, put them next to a banana? Or a nuclear reactor: this produces an effect much more quickly. Indeed, the presence of Fe3+ impurities alone will give the quartz a purplish hue but this can fade over time whereas the irradiated purple colour is permanent.

The word amethyst originally came from the Greek for "non-intoxicated" as in Greco-Roman times it was believed that amethyst could be used to fend off drunkenness and wine goblets and tankards were carved from amethyst and drinkers wore amethyst pendants and bracelets to stay sober.

Until the 18th Century, amethysts were highly valuable but around this time large deposits were discovered in Brazil which revalued this gemstone to the value it holds today.

Where did the structure come from?

This particular representation of the quartz structure comes from a 2008 high resolution XRD redetermination.

Tags: birthstones   quartz   purple