Play-of-Colour — The Diffraction Phenomenon at the Heart of Precious Opal
Play-of-Colour — The Diffraction Phenomenon at the Heart of Precious Opal
How silica spheres separate white light into the spectral flashes that define gem opal
Play-of-colour is the optical phenomenon in precious opal characterised by flashes of spectral colour that shift with viewing angle, and is the single feature that separates precious opal from common opal. The cause is the diffraction of white light through a three-dimensional lattice of uniform amorphous-silica spheres of submicrometre diameter, with the lattice acting as a natural diffraction grating in three dimensions. Play-of-colour is the value driver of opal at every level of the market and is the criterion against which fine opal is graded.
The physics
Precious opal consists of nearly equal-sized spheres of amorphous silica — typically between 150 and 400 nanometres in diameter — packed in a regular three-dimensional close-packed array, with water and air filling the interstitial voids. When white light enters the stone, the spacing between adjacent layers of spheres acts as a diffraction grating: certain wavelengths are diffracted constructively in particular directions, producing a flash of pure spectral colour at the angle where the geometry resolves.
The size of the silica spheres determines the wavelengths that diffract effectively. Smaller spheres in the 150 to 200 nanometre range produce blue and violet flashes; mid-range spheres around 250 to 300 nanometres produce green and yellow; larger spheres around 350 to 400 nanometres produce orange and red. Stones containing a range of sphere sizes can show the full spectral range of flashes; stones with uniform sphere size show a more restricted range of colours.
Patterns of play-of-colour
The arrangement of the diffracting domains within the stone produces characteristic patterns of play-of-colour. Harlequin, the rarest and most valuable, shows large square or polygonal patches of distinct colour packed across the face of the stone. Pinfire shows tightly packed small flashes resembling pinpoints of coloured light. Broad-flash shows large sheets of colour that change as the stone moves. Floral, ribbon, rolling-flash, and other named patterns describe particular geometries; the trade vocabulary for opal pattern is extensive and varies between Australian and Ethiopian production.
Pattern, range of colours, brightness, and the proportion of the stone showing play-of-colour all contribute to value. Red play-of-colour is typically the most prized because it is rarest and hardest to produce in nature; blue and green are most common.
Body colour and origin
Play-of-colour is independent of body colour. Black opal from Lightning Ridge in New South Wales shows play-of-colour against a dark body that intensifies the perceived colour saturation; white opal from Coober Pedy in South Australia shows the same diffraction phenomenon against a light body, with the flashes correspondingly less dramatic; fire opal from Mexico has an orange-to-red body that may or may not exhibit play-of-colour. Crystal opal is transparent-to-translucent and allows play-of-colour to be seen against background light.
Ethiopian opal from Wollo, discovered in commercial quantities in 2008, has expanded the supply of play-of-colour material. Ethiopian opal is hydrophane — it absorbs water and changes optical character in response — which is a critical handling consideration but does not affect the fundamental play-of-colour mechanism.
In the trade
Buying opal demands attention to play-of-colour first and to body colour, transparency, and treatment second. The most valuable stones combine high brightness, broad pattern, the full spectral range, and either a black or crystal body. Treatment is a routine concern: smoke treatment, sugar-and-acid treatment, and resin impregnation all produce stones that look more saturated than the natural rough warrants and must be disclosed. Origin reports from competent laboratories address treatment status when commissioned.