Air bubble gems

Beautiful gemstones are always popular with the public when they’re brought out for Spotlight Specimens. Monica Price talks about some she’s been showing off recently at our daily drop-in sessions.

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Now here’s a mineral that most people recognise straight away when I bring it out for Spotlight specimens. It is amethyst, and it is the most popular of all purple gem minerals. By a happy coincidence it is very common too, so jewellery made with amethyst need not be very expensive.

But that wasn’t always the case. In the past, fine, large, transparent crystals of amethyst could only be found in Germany, Russia and a few other places in the world. In Europe, the colour purple is traditionally associated with royalty and wealth, and so rare amethyst gems would feature in crowns and jewellery worn by heads of state and religious leaders.

Amethyst is actually a variety of one of the Earth’s most common minerals, quartz.  Quartz is composed of silicon dioxide and helps form many different kinds of rock…. it even makes up most beach sands! Amethyst is the kind of quartz that contains a little bit of iron to turn it purple.

So how did amethyst suddenly become so common? During the 18^th century, huge flows of volcanic lava were found by explorers in Brazil and Uruguay. Some had air bubbles which were lined with superb purple crystals of amethyst. Soon, these crystal-filled cavities were being sent to Europe, and today, they are sold all over the world.  The biggest bubbles were a metre or more in size – huge! Nowadays, nearly all the amethyst you see for sale comes from those 135 million year-old South American lava flows.

My spotlight specimens include an amethyst gemstone and some lovely examples of those gas bubble cavities lined with crystals. One rather curious thing is that the crystals are rarely purple all the way through. The colour typically concentrates towards the tips of the six-sided crystals.  If you come when I next show my ‘gas bubble gems’, you will see exactly what I mean!

Monica Price, Head of Earth Collections