The origin of the colour in opal has been met with many theories. However it has now been demonstrated that the regular array of spheres and voids in the opal diffracts white light by breaking it into the complete range of spectral colour. The colour observed is primarily dependent on the layer spacing, which is determined by sphere size.
The colour observed also depends on the angle the light hits the spheres and the position of the observer. This can easily be demonstrated by rotating a red-fire opal and seeing one particular area change from red, orange, yellow, green, blue, purple as the angle of incidence to the observer is increased. Green opal will only show green to blue colours on rotation as the sphere size controls the highest order colour observed. A blue opal when rotated will only show blue, purple to black colour as the sphere size does not produce the higher green or red colours.
If the silica spheres are cemented together more than spheres aligned in an regular pattern, the porosity is greatly reduced, therefore light passes straight through the specimen without being diffracted to produce colour. The end result is just clear potch.
In potch opal, there is no play of colours, the silica spheres are either too small to even produce the blue colour even when arranged in a regular pattern, or the silica spheres are of an assortment of sizes and do not produce the regular array required for colour diffraction.
Potch with small colour bars
Why is some opal called a living stone? Faults in stacking or growth of silica spheres create grain boundaries which result in differences and abrupt changes in colour at these boundaries. In natural opal, each grain boundary is sharp and straight sided, and the texture of each colour flake is uniform. In synthetic opal, blurred boundaries are apparent and sub-grains can be seen inside most colour grains. Synthetic opal generally has repeating colour patterns and not as random as natural solid opal.
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