For decades the exact process that led to opals’ various displays of colour was a mystery. Many assumed it was a similar process to other gemstones. Like diamonds or sapphires, who’s colour is caused by light bending inside the crystal lattice (refraction) or impurities lending them their colour.
However, opal is one of only a handful of stones to instead create its unique fire through light’s diffraction.

Diffraction, broadly

First off, before understanding how this is possible in opals, we must understand what diffraction is. Diffraction is simply the way that waves move around objects; in this context a wave could be the type you’d find at the beach, a sound wave, or even light. The two examples below demonstrate how this phenomenon works on two very different scales.

This is a snapshot from Google Earth over a harbour in Eastern Alexandria. It shows how waves approaching a harbour are affected by diffraction and will fan out after passing through a narrow gap in the harbour wall. As they pass through this gap the energy of the radiating waves is dispersed across the entire harbour.

The diagram depicts this exact same principle as what’s occurring in the harbour, however on a much smaller scale. With the ray of white light approaching and then passing through a proportionally sized gap it too will fan out and disperse its energy. Therefore, since white light itself is actually made up of many different wavelengths of light these will diffract in different ways. This effectively splits the the once singular white ray into its many component colours.

Diffraction in Opal

This process of diffracting light is something that happens billions of times in an opal. How is that possible? It's because opal is just an amorphous collection of silica, that on occasion can organise each of its spherical particles into a closely ordered pattern. The images below show what an opal looks like at the microscopic scale, with the help of a Scanning Electron Microscope (SEM). Here the underlying structure of an opal is revealed to be billions of these tiny silica spheres all with approximately the diameter of a virus (150-300 nm) stacked on top of and next to each other.

Since opal is made up of these tiny spheres at a fundamental level, when they are stacked gaps or voids are created between the spheres. These voids are what allow the process of diffraction to occur within the internals of the gem. Additionally, when these gaps are regular in size and position like in the above images, then the light undergoes a second type of diffraction that increases the lights apparent intensity, making the effect visible to the naked eye, leading to a bright flash of colour. When opal, or any other material, acquires this structure it is known as a diffraction grating.

In this diagram there are now two gaps for the wave to pass through. As each gap causes its own diffraction pattern to occur the waves will interfere with each other. Where their peaks cross the intensity (brightness) will increase and, at the same time, when a peak and a valley cross they will cancel each other out leading to no intensity at that point. This creates an intricate interference pattern fanning out in all directions.

Placing this back into the setting of an opal, imagine this effect occurring for each and every void in the silica sphere stacks. The many rays pass through each individual void in the opals structure and diffract, they then interact with each other further through constructive and destructive interference. This explains why rotating an opal will change the perceived colour. Because the light is having to travel through a different set of spheres and voids, causing a different amount of diffraction and interference. With the light finally exiting the opal with a now totally different colour.

You can see this in the accompanying video of a white opal Notice how while it’s rotated the colours appear to dance across the face of the stone, not staying fixed in any one place.

Effect of Silica Sphere Size and Spacing

The size and regularity of the silica spheres here is crucial since smaller spheres will lead to smaller gaps and thus smaller wavelengths of light. These spheres are much more common to find within the 150-250 nanometer range leading to small wavelengths such as blue to be most common in precious opals. Therefore, due to their rarity, colours with larger wavelengths like reds, yellows, and oranges are more sought after, usually demanding a higher price.

This opal is a harlequin opal, so named for its distinct and striking contracted diamond pattern of blue and red. This variety of opal is one of the rarest out there thanks in part to the proximity of the blue and red fire. These two colours are on opposite ends of the visible spectrum and thus require the opal to have very different sized silica spheres to be produced, red with larger and blue with smaller. Having such a stark transition between the two is very uncommon.

Now, what if the structure of the spheres is irregular and mismatched in size? Then it becomes potch aka common opal, a murky mostly colourless version of opal. This is what miners find the most of, since, it is significantly more likely for opal to form without any order or regularity. The light is still able to pass through the gaps in the silica but it all diffracts to different wavelengths, and therefore different colours.  Making the light not form a clear distinct colour by interfering and mixing with itself. Thinking of it like mixing too many different colours of paint it mostly just leads to a nonspecific grey.

Opal is one of those gems with no equal. Thanks to a complex interplay of the physics of diffraction and the geological processes that formed opals in the first place. This one gem gained the ability to produce a countless variety of patterns & colours found no where else in nature.

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