Opal is a form of silica similar to quartz but containing water within the mineral structure. Precious opal contains between 6 and 10 percent water and is composed of minute silica spheres arranged together in a regular array.
The size and spacing of these spheres is what determines the colour and intensity of colour we see in opal.
The main factors determining the value of opal are:
1. Type of Opal
The type of opal is very important when determining the value of opal. The types of opal are: Light/Crystal, Boulder, Black, Treated Matrix, Doublet and Triplet Opal. A boulder opal may have a very similar display of colour to black opal but the black opal will command a higher price.Doublet and Triplet opals are a composite containing thin slices of opal stuck to other materials to enhance their colour and will fetch a lower price than a solid with the same colour.
Red and orange is generally more valuable than green or blue but a good variety of colour with predominant red is most desirable.
The brilliance or brightness of an opal is of utmost importance. An opal with less variety of colour that’s glows brightly will get a higher price than a stone with every colour that is dull by comparison.
4. Clarity or Transparency
Light opals are more desirable when they have good transparency with bright colours. This is what is called crystal opal and is the most prized of light opal.
In black and boulder opals the darkness of the body tone is a factor in determining their value. A black body tone is preferable to grey or lighter body tone.
A larger pattern of colour is preferable to a smaller pattern.
Pinfire and patterns where the colour is in small specks will fetch a lower price than large broad patterns or flashes of colour such as harlequin or rolling flash.