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 0pal.
Opals generally fall into one of five types including; Black, Dark, Light, Boulder, and Matrix. For an broader overview of opal history and the types of opal, please refer to the Opal History and Types section.
The type of opal can be further sub-classified according to variety. Black, Black Crystal, Semi Black, Dark Grey, White, Crystal, Light Grey, Jelly, Boulder, Boulder Matrix, Yowah Nuts, Andamooka Matrix etc
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.
Transparency refers to whether it is opaque, translucent or transparent. That is, how much light can pass through the opal and with Light Opals, transparency can be tested by placing a black dot at the back of the opal and determining whether it is visible from the front.
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.
Directionality is determined when looking at the stone while it is rotated through 360° in 90° increments and observing how the colour is displayed and how the hues are arranged. A lot of opals may ‘flash’ from one direction yet appear clear from another with most displaying their colour when rotated to a certain angle, while the finest opals are non-directional.
Distribution refers to ‘depth of the pattern and the amount of ‘fire’ is shown on the face. Sparse distribution yields a lower value.
As the name suggestions, saturation refers to dominance of hue in the fire colour with rich saturated hues being present and vivid in the better quality stones.
Fire (Play of Colour)
Fire refers to the dominant hues in an opal and are propended to the particular hue of the stone, e.g. red-fire. In most opals, the spectral colours (red, orange, yellow, green, blue, indigo, violet - in order of value) are combined to some degree and the predominant hue should be atleats 50-70% with the secondary hue coming in around 20%.
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 dig 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.
Not all opals are the same nor do they all come from the same opal field. This can also have an influence on the value of the opal. Australia’s opal fields include:
New South Wales:
- Lightning Ridge
- Sheep Yard
- White Cliffs
- Coober Pedy
The ‘weight’ of an opal is measured in terms of Carats; 1 Carat equals 0.2 Grams. Prices of opal are best for stones that are 2-5 Carats (10-25 grams) and up to 10 Carats (50 grams). opals bigger than this, while prized, are less viable for jewellery purposes and the value/carat can be reduced.