The Science Behind Australia's Magical Gem: The Opal
Did you know that the opal is Australia's national gemstone? Around 95% of the world's opals come from our country. These beautiful gems possess one striking feature: when you move the stone in your hand, the coloured patterns on the gem transform and move. Experts call this phenomenon "play of colour."
Play of colour distinguishes valuable precious opals from common opals (also known by the miners' term "potch"). And because the colours contrast better against a dark background, black or dark opals sell for an even higher price.
But where does the seemingly mystical play of colour in precious opals come from? If you've seen an opal in either your own collection or at a jewellery store, you've probably asked that question.
Let's take a look at the science of diffracted light and the microscopic structure of opals to find the answer. We'll also find out what determines the different colours in each individual opal.
Diffraction of Light in Opals
The shifting colour in opals results from a scientific property called diffraction. Light travels in waves. When a wave collides with a small obstacle or opening, the wave bends. This process is known as diffraction. When ordinary white light passes through a structure with many small openings (called a diffraction grating), the white light sometimes bends and changes colours.
You've seen light diffraction today if you looked at the back of a CD or DVD. White light becomes diffracted in the disc's narrow grooves, creating rainbow patterns as you move the disc back and forth. In the same way, opals diffract light because of their structure, producing their signature multi-colour shine.
An Opal's Unique Structure
Although Australia's gemstone shares the beauty of other precious gems, opals differ from other gems in terms of structure. Opals don't follow the crystalline shape that other gems are known for. Instead, opals are a non-crystalline combination of water and silica. The presence of water separates opals from their close cousin, quartz.
The silica in opals takes the form of microscopic spheres. To visualise the structure of an opal, imagine pouring hundreds of tiny pearls in a glass cube. These spheres group together as tightly as they can, creating the structure of an opal.
If the size and arrangement of these tiny spheres are random, it becomes a common opal. In contrast, the silica spheres of a precious opal have the same size and shape. When light shines through the silica of a precious opal, the light bends at just the right wavelengths to create new colours.
The Different Colours in Opals
Some opals might reveal only one colour, while others seem to contain an entire rainbow. What causes this difference? The answer, once again, comes down to those tiny spheres of silica.
The size of the spheres determines the available colours in the opal. Small spheres generate a blue colour, which is also the most common colour in opals. As the spheres get bigger and bigger, other colours follow, including green, yellow, orange, and red. So an opal with two colours, blue and green, means that it contains the next smallest spheres, with blue-only spheres being the smallest.
The largest spheres can produce red in addition to all the other colours. For this reason, opals containing red are the rarest and also the most valuable.
With all the outrageous circumstances required to generate a multi-coloured precious opal, it's no surprise that these gems possess a remarkable value. Hopefully you can take pride in Australia's national gemstone. And now you will understand the magical gleam of an opal the next time you see one at a jewellery store.