The February Birthstone – Amethyst

Welcome to blog post number two about birthstones (again, published in the last second). As you know I like to get science-y with these posts and probably use terms and jargons that you’ve never heard of. But I will try and explain everything as simple as possible for you. After all, I want you to understand what I’m talking about, so you can see why gemstones are so much more than just pretty minerals adorning the jewellery we wear.

 

 

How to not get drunk

February means it is all about the amethyst! Amethyst is, as many of you may know, the purple variety within the large quartz family with a simple chemical composition, namely SiO₂. Since it’s a quartz variety, it has a hardness of 7 which means it’s very suitable for jewellery since it doesn’t scratch easily from everyday wear.

Amethyst got its name from the ancient Greeks who thought amethyst would keep them from getting drunk. The word amethystos literally means “not intoxicate”. They would wear or drink out of amethyst vessels in the belief that it would prevent intoxication.

 

Left: carved amethyst wine goblet. Right: my amethyst earrings in sterling silver.

 

Ultra Violet

If you didn’t know, the Pantone colour of the year for 2018 is Ultra Violet, which suits the deep purple colour of the amethyst pretty well if I do say so myself. Amethysts come however in many different purple hues, from a light pinkish violet all the way to that royal deep purple. The most valued colour of amethyst is the deep purple one and a deep purple that has a red tinge to it.

So how does amethyst get its purple colour? Simply put it all has to do with the presence of iron in its structure. Iron atoms may replace silicon atoms during the crystal’s formation. And we are talking trace amounts here, the amethyst wouldn’t be an amethyst anymore if a major part of the silicon atoms were to be replaced.

One thing that you should be aware of when it comes to the colour of the amethyst is that if it gets overexposed to sunlight, it can fade in colour. So, if you want to keep that deep purple colour of your amethyst jewellery or mineral specimen pristine, display or store it in a place with little to no sunlight.

The ametrine

 

A rough ametrine specimen with a clearly visible border between the amethyst and the citrine halfs.

 

Have you ever heard of the gemstone ametrine? The ametrine is a gemstone that is half amethyst and half citrine, which means it displays both purple and yellowish hues. This dual colouring is due to differing oxidation states of the iron within the crystal. And the differing oxidation states occur during the crystal’s formation (its crystallisation phase), as the temperature gradient across the crystal cools over time.

The formation of an amethyst

Amethyst crystals are always grown onto a base within a cavity of a host rock. This sort of rock with a sparkling surprise inside is called a geode.

 

Amethyst geode from Brazil. The crystals are grown onto a base consisting of agate.

 

Geodes are round to elongated rock structures which are hollow inside with minerals lining the walls. The base of the mineral lining is usually thin bands of translucent grey and white agate which the crystals then grow on top on. Geodes range in size from under one centimetre to several meters in length. From the outside most geodes look like common rocks, but when they are opened the sight can be breath taking. The outer wall of a geode is more resistant to weathering than the surrounding bedrock which allows it to survive intact when the surrounding bedrock weathers away.

Apart from amethyst crystals, other common geode minerals include quartz, agate and calcite. Some rare geodes can be filled with beautiful blue gem silica, pink rhodocrosite, spectacular opal or other rare materials.

 

Two beautiful geodes with spectacular mineral landscapes within. To the left: a basalt geode with mesolite and chalcedony. To the right: a basalt geode with blue chalcedony, calcite and mordenite bobbles. Both from India.

 

The formation of a geode

I bet you wonder how on Earth there can exist hollow cavities within solid bedrock, so I will tell you. Most geodes are found in volcanic rock deposits such as basalts. These are rocks that normally contain a lot of gases and it’s the gases that creates these cavities. When the lava cools and solidifies, any gases that hasn’t been able to escape gets trapped and thus create cavities within. Another way a cavity can form in volcanic rocks is by liquid lava flowing out of a partially solidified lava flow. These ‘lava tube’ cavities produce some of the largest and longest geodes.

 

Several amethyst geodes within an ancient basaltic lava flow.

 

Geodes can also be found in sedimentary rocks such as limestones, dolomites and calcareous shale. Within these rocks, shells, tree branches, roots and other organic materials often decay away to leave a void. These geodes are generally smaller than the geodes formed in volcanic rocks.

What all of these cavities has in common is that their mineral lining is due from mineral rich hydrothermal water or groundwater finding their way in and precipitating mineral material over a long period of time. And it’s the mineralogy of the surrounding rocks and the minerals within the water that decides which minerals will form within the geode.

So, if you want to find some amethyst crystals yourself, look for geodes within old basaltic lava flows!

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So that’s that – I hope you enjoyed this blog post and that I managed to make you look at an amethyst in a completely new way!

Linda