The beauty of an opal is unique which is is not only derived from its ‘play of light’ characteristics that displays spectral colours like no other gem stone. Similarly, the characteristics in classifying the quality of an opal is equally unique.
Diamonds adhere to a 4C’s group of determinants - Colour, Clarity, Cut, Carat, but for opal, there are nearly four times the number of elements that can be considered when determining the quality of an Opal.
Opals generally fall into one of five types including; Black, Dark, Light, Boulder, and Matrix.
Body Tone is the first determinant in regard to quality and price and refers to the ‘tone’ which can range from colourless, white, and grey to black. The dark to black background being generally more desirable than a grey or milky-white background.
Vividness of colours is of paramount importance with the brightness of an opal being directly related to price. As several levels of brightness cane be observed in the face, it is the overall brightness which used to classify it as Subtle, Bright, or Brilliant.
Distribution refers to ‘depth of the pattern and the amount of ‘fire’ is shown on the face. Sparse distribution yields a lower value.
There are many different patterns that can be used to describe an opal, and each has a great influence on price. In general, the larger the pattern, the great the value. In addition, the more vivid the pattern, that great the value for that attributed pattern.
Shape of the final cut of opal is most often determined by the shape of the rough form as dug from the ground. Most opals are cut in a more oval shape, and is non-faceted (en cabochon) with a dome face as that is the beloved to provide the best display of the opal’s characteristics and is also shape preferred by jewellers.
Also sometimes referred to as the ‘balance of the cut’, proportion refers to size of the dome in relation to the colour bar with the ‘play of colour’ to the thickness of the potch or host-stone. Cut too thickly, the stone will be unbalanced by not enough spread/face for the size of the host-stone
If a stone has scratches or cracks within the stone it can dramatically reduce its value. Inclusions may include; patches or lines of potch, 'webbing', 'sand spots', crystals of gypsum and ironstone in the face of Boulder Opal. In Black and Boulder Opals, ‘windows' where there is an area of transparency within the body of the opal that will allow light to enter through the back of the stone resulting in the dilution its play-of-colour. These windows will also reduce the value of the opal.