Opals – pretty things

I have an opal pendant that a friend brought back for me a few years ago from her trip to Australia. It looks like cracked pale blue ice on top of purple and green lights. Colours shift as the viewing angle or lighting changes. Some believe that opals have healing powers and can help you find your true love -- I'm not so sure about that, but I do think opals are pretty examples of some neat optics.

The opal's popularity peaked with the flappers in the 1920's. The artisans of the time catered to art deco ideas, a style seen as glamorous and modern. They valued opals because of their subtle effect and how nice they looked with enamel, another popular material of the time. Ironically, my grandmother gave me some costume jewelry from that period that doesn't have a single opal. Since then, opals have been studied under electron-microscopes, made in labs and even found on Mars. The name opal likely comes from a Sanskrit word upala meaning 'valuable stone', a term adapted into Greek and later into Latin.

Opals have been mined dating back to Roman times, and probably longer than that. Currently almost 95% of all opals come from Australia, while other sources include Mexico, Brazil and the United States. New sites are still being found. Every opal site produces opals that are slightly different, being a different colour pattern, clarity or luminosity.

So what is so captivating about an opal? A single stone can display all the colours of the spectrum in moving patterns depending on the viewing angle – this movement across the opal face is called the 'play of colour'. These effects can even take on an iridescent quality that is impossible to reproduce (I tried to capture these effects with my camera and it didn't work). If you look through the stone and see red (a rare thing), all the other colours will also appear in that stone because red is at the long end of the colour spectrum (630-740 nanometers) and can be broken into all the other colours. Finally, opals can have a translucence ranging from transparent to a milky opacity. The colours found within an opal are formed through the interference and diffraction of the incident light as it passes through the internal structure.

If you examined the micro structure of an opal you would see silica spheres of 150 to 300 nanometers packed together in a not-quite regular lattice. An opal isn't a true crystal due to the its non-regular lattice, instead it is classified as a 'mineraloid'. The quality of an opal can be determined from its structure – the more regular it is, the more valuable it is. Another aspect to notice at the microscopic scale: namely, pockets that were once filled with water. These pockets affect the optics and are a remnant of how opals are formed.

Opals are formed from dissolved silica in water. Let's start with dissolved silica in a bucket sitting in the sun; over time the water will evaporate and if new water isn't added the silica molecules will bind together, forming small spheres. Eventually, if the bucket is left undisturbed for eons, the spheres will become tightly packed and will form layers. More water will evaporate as the layers compact and harden, leaving tiny spaces where the water once was.

Light waves range in size from about 400 to 760 nanometers – about twice the 150 to 300 nanometers of the silica balls. As light enters an opal, the the silica spheres and the water voids diffract or absorb some wavelengths, changing the composition of the light wave and hence the colour of the light reflected by the opal. By changing your viewing angle the light path through the opal to your eye also changes creating new patterns and colours. The most spectacular and likely most expensive opal is the fire opal. Here the silica spheres are lined up so well they act like a diffraction gate, breaking down the incident light into a full rainbow of colours.

Remember the water inside an opal? Well, it isn't completely gone, 2 % or more remains. That little bit of water is required to keep the opal from becoming brittle and paler. So to keep your opals pretty you have to wear them; the humidity in the air and on your skin will keep them looking their best.