Opal Valuations
There are many variables determining an opal's value which have to be considered by professional valuers. The following are the four main factors.
Body Tone
Opal body tones vary from "black" (N1 - N3) through "dark" (N4 - N6) to "light" (N7 - N9). The darker the body tone, the more valuable the opal is. The reason prices change for the different body tones is twofold. Firstly, the colours in opal having a dark body tone are far more vivid and, secondly, because opals with a dark body tone are rarer.
Colour
From the research done by the CSRIO in 1964 with the electron microscope, we know the colours in opal are achieved by the path light takes passing through uniformly sized silicon spheres (and the voids between them) packed in an orderly three-dimensional grid.
The possible colours are determined by the size of the spheres. Smaller spheres 150nm in diameter produce the smaller wavelength colours of violet and blue. As the size of the spheres increases so do the possible colours up to 300nm where red, the upper limit of the visible spectrum can appear. The value of the opal colour is determined by its rarity, smaller spheres are more common than the larger spheres and so the blues and greens are less valuable than gems having yellow, orange or red in them. Opals can display a single colour, two colours through to gems giving off every colour possible. It is worth noting that the rarest (and so most expensive) of colour combinations is bicoloured red and blue/violet because this is an opal capable of displaying both ends of the visible spectrum with the intermediate colours absent, these are extremely rare.
Luster
When considering the Luster or commonly called brightness of an opal it is elementary that the brighter the stone, the more valuable it is. The technical reason why opal from Lightning Ridge and Coober Pedy appear brighter than opal from Mexico is due to the different types of opal and how it was formed. Lightning Ridge and Coober Pedy opals are sedimentary whereas the Mexican opal is volcanic. In volcanic opal, there are smaller spheres present between the larger spheres which scatter the light and so it loses intensity whereas in sedimentary opal the voids between the spheres are empty allowing the light to pass freely and so keep its intensity. It is hypothesised that the relatively quick formation of volcanic opal compared to the slower sedimentary opal formation is the cause of the voids being filled with smaller spheres in the volcanic opal.
Pattern
Unlike synthetic opal, every natural opal is unique. Not only is it unique but every angle of a natural opal can show a very different pattern. Generally speaking, the larger the un-interrupted areas of individual colour the higher the rarity, quality, appeal and so the value. If the pattern appears to repeat within the stone the value is greatly increased. As the opal is turned it can give a flash of colour which is desirable, even more highly prized is when the flash appears to move across the stone and sometimes changes colour as it moves, this is called a "Rolling Flash". There are many pattern names - Harlequin, Ribbon, Script, Chaff (or Straw), Flagstone, Floral, Moss and Pinfire. Of these Moss, Pinfire and Floral patterns are the most common. Extremely rarely a picture can be made out, be it a bird, a ship or anything else recognizable. Understandably this will allow the stone to command a much higher value.
Inclusions or Faults
Being a naturally formed stone, very few are without any imperfections at all. Some minor detraction in values are determined when the weight is below one carat, has a shallow dome or has a suboptimal depth all while still being within tolerance.
Some buyers prefer to have a "clean" back while others prize some sand visiable adding to the uniqueness and natural texture. Provided the sand does not weaken the stone and is not visible from the face it has negligible effect on the value.
The same is true of the freeform stones as opposed to the standard round and ovals. With the lower quality smaller stones which might be set in mass-produced settings, it is more desirable to have the stone calibrated so it can be set economically. With the higher quality stones where the owner will either be keeping them for investment or having them put into a custom setting, the shape adds to the distinctiveness of the stone and can add to the attractiveness and value of the finished jewellery.
More serious faults include "windows" where there is a gap in the background material, potch in the face of the stone which interferes with the pattern. However, sometimes a border of black potch separating blocks of colour can actually add to the attractiveness and value of a stone.
Stones with Matrix, gypsum webbing or cracks need to be re-cut before being valued.