Valuing Opals

The factors in pricing Opal are by the 'Play-of-Colour'. The more brightness the gem produces will increase its value, along with spectral colours, which are the actual colours seen in the 'Play-of-Colour'. For example, Opal with Red colour is considered more valuable than Opal with Green colour play. Green is then considered to be more valuable than Blue.

The more pure or vivid colour saturation of the Opal will also increase the value. Patterns in Opal are also a factor to consider. Size, shape and regularity of the colour patterns is also important. The direction of the colour and pattern is also highly notable. When rolling the gem from side to side, see if the colours are visible in all directions.
Inclusions in the Opal such as Potch patches or Potch Lines will effect the value, so determine what they are, how many and their position within the face of the stone. Carat weight of the gemstone like other gemstones plays a part in its value.

The Australian Opal and Diamond Collection does not stock or sell any treated, colour enhanced or synthetic opal.

Diffraction

Depending on the size of the spheres, varying colours of the spectrum are diffracted. So it is a combination of deflection (bending) and diffraction (breaking up) of light rays that creates the colour in opal.

If you move the stone, light hits the spheres from different angles and brings about a change in colour. The name opal actually means "to see a change in colour." The way in which colours change within a particular stone as it is rotated and tilted is called the stone's play of colour.

The colour Red is most valued, followed by Green, and lastly Blue.

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.