How Opal Color is Produced [see also www.opalmine.com encyclopedia ‘how opals glow color’ ]
It took the development of the electron microscope to work this out. Precious opal is made up of tiny uniform spheres of transparent hard silica, which fit together in an orderly three-dimensional frame, sitting in a ‘bath’ of silica solution. It is the orderliness of the spheres that separates precious opal from common opal. Light passes through the transparent spheres in a direct line, but when it hits the ‘bath’ of silica, it is bent and deflected at different angles, thus producing a rainbow effect. Depending on the size of the spheres, varying colors of the spectrum will be diffracted.(see definition below) So it is a combination of deflection (bending) and diffraction (breaking up) of light rays that creates the color in opal. If you move the stone, the light will hit the spheres from different angles and bring about a change in color. (Which is what the name opal actually means.)
common potch
precious potch
It is of interest to note that the size of the spheres has a bearing on the colour produced. The smaller spheres bring out the blues, from one end of the spectrum. The larger spheres produce the reds from the other end. The more uniform the spheres are placed, the more intense, brilliant and defined will be the color.
Where Opal is found in Australia see www.opalmine.com-encyclopedia ‘the miners terrain]
To this point, nearly all the opal in the world is found in Australia. It has been recently named our national gemstone.
Because of it’s very dry inland climate, a greater percentage (but not all) of this opal is crack resistant and better suited for jewelry making. It is difficult to predict which opal will crack and which will not.
It can be generally stated (but not absolutely) that Light colored opal (crystal and laminates such as doublets and triplets) comes from the South Australian fields (S.A.), Black opal comes from the Lightning Ridge, New South Wales. (NSW) area, and Boulder opal comes from Queensland (Qld).
Having said that, it is true that the three fields all produce some light and dark opal. The exception to this is that Queensland is the only state producing boulder opal at this point in time, and very little black opal is found there. Some boulders, which are exceptionally dark, are referred to as black opals, which they are, but not in the same sense as Lightning Ridge black opals.
New South Wales. (NSW) area, and Boulder opal comes from Queensland (Qld).
Having said that, it is true that the three fields all produce some light and dark opal. The exception to this is that Queensland is the only state producing boulder opal at this point in time, and very little black opal is found there. Some boulders, which are exceptionally dark, are referred to as black opals, which they are, but not in the same sense as Lightning Ridge black opals.
Faceting takes a lot more expensive equipment, is more difficult to learn, takes a lot of time, and in most cases the efforts are not rewarded because the job is already being done very cheaply by large well organized factories using cheap 
Note: There are quite a lot of differences between the Australian and American ways of spelling. I have opted in most cases to using the American, since most of my business is done in that country,. . and my word processor speaks American and keeps telling me that I’ve made a mistake…so I’ve decided to go along with it. Also keep in mind that for every suggestion made about gem cutting procedure, there will be someone who disagrees with it. If you think you’ve got a better suggestion, or would like to put your point of view, please take advantage of the chat program on the 
Unless you understand the various terms associated with the movement of light, you will not understand how various opals are identified and put into different categories or groups.
As an opal dealer, you will want to flatter your stones and present them in their best dress so as to speak. 
