Opal and Play-Of-Colour — Why Are There So Many Colours?
Opal displays all the colours of the rainbow. It is the structure of opal that gives us all the colours of the visible spectrum. Why is this play-of-colour seen in some opals and not others?
Composition and Structure of Opal
Opal is an amorphous hydrated silica. This means that opal does not have a crystal structure like mineral gemstones and it contains water. Opals can crack and show crazing if the water content dries up.
Opal is composed of tiny spheres of silica arranged in a closely packed, orderly, three-dimensional array. This arrangement of spheres has empty spaces between them occupied by air and water or silica of a different refractive index from the silica spheres. These sphere range from about 150 nanometers to 300 nanometers in diameter.
What Happens When Light Hits Opal
When the light passes through the basically colourless opal and strikes the planes of voids (spaces between the spheres), certain wavelengths are diffracted and flash out of the stones as nearly pure spectral colours. This display of colour in precious opal is called play-of-colour.
Where the distance between the regularly packed planes of spheres is approximately half the wavelength of a component of visible light, the light of that wavelength may be subject to diffraction from the grating created by the stacked planes. The spacing between the planes and the orientation of planes with respect to the incident light determines the colours observed.
The size of the spheres (therefore the size of the spaces between the spheres) determine the colour and the number of colours that will be seen. The larger the spheres the longer the wavelengths, red can be seen, as well as the colours of the shortened wavelengths. Smaller spheres will only show a blue colour. Blue opal is less expensive than opal that shows reds and a combination of many colours.
Do We Like Common Opal showing Opalescence?
If opal does not show a play-of-colour it is called Common Opal. There are no colours displayed because of the random packing of the spheres of irregular size. Common Opal is also known as “potch”. The term opalescence is commonly and erroneously used to describe the unique and beautiful phenomenon, which is correctly termed play of colour. Contrarily, opalescence is correctly applied to the milky, turbid appearance of common or potch opal.
There are enhancements for natural opal as well as synthetic and imitation opals on the market. The appearance of natural opal may be enhanced by a smoke and sugar treatment, surface oiling and plastic impregnation. Opal slices are also used to make opal triplets and doublets. All of this contributes to many different price points for purchasing opal. If you are contemplating purchasing an expensive opal it is worth investigating all of the varieties of opal. Every Opal is different and every person is different! There is a beautiful opal just for you.
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I plan to introduce different gemstones alphabetically. I started with “A” (Ammolite) and it has taken me to Malachite! Leave a comment and let me know if you have a favourite gemstone that starts with the letter “O”. Take the poll! Can you name a reference to a gemstone in a song, movie or book?