We all know of and love the October birthstone of opal. It is easily recognizable by most people, with its spectacular rainbow display which bounces around in light and creates the most hypnotic patterns and dances. Every single opal is individual and unique to its other opal siblings, simply because of this spectacular 'play of colour' optical effect – but what is the science behind this, and why are its colours so unique?
Opals are formed when silica rich water travel deep within the Earth and settle within cracks and crevasses within the crust. When it rests into a gel and hardens, the water evaporates out of the mix and leaves behind silica spheres.
These silica spheres stack on top of each other and vary in placement and size. Imagine a bucket full of ping-pong balls; these fall on top of each other randomly and with varying gaps between each one. Opals are not dissimilar to this as an amorphous material, having a non-symmetrical and random crystal lattice. Within the opal structure, these silica spheres can aggregate together forming an orderly, symmetrical pattern within the amorphous structure.
When light passes through these spaces within the crystal matrix, it diffracts and creates these amazing displays of colour. Diffraction of light occurs when white light waves pass through minute openings or slits within surfaces. This then bends the light and fans it out, allowing the spectrum of light to spread. This effect causes our eyes to see the individual colours of the rainbow, or only certain colours at one time.
With the varying amounts of aggregated silica spheres, at varying sizes and distribution, light can diffract throughout the nanometer-wide gaps within the stone causing a fabulous display. This optical effect is called a ‘play-of-colour’, and is what makes opals so unique.
Smaller gaps within the stone produce blue and green colours within the stone, and larger gaps produce yellows, oranges and reds. These larger gaps are uncommon within opals, which makes red and oranges so rare in comparison to the blues and greens. Red and orange colour plays open a black opal are the rarest of these gemstones.
The varying levels of play-of-colour within opals determine how precious they are. Those opals which are opaque and show limited play are defined as ‘common’, and can be found in multiple places across the world. Those opals which have a great level of play and show those beautiful reflective patterns are defined as ‘precious’, with the finest specimens being found within Australia, Ethiopia, and North America.