Australia is the world’s largest producer of opal, with over 90% of all the world’s opal being mined here. Opals are found in the outback arid regions in three different states of this very dry island continent.
This is just a very brief summary for the lay person wanting an introduction to Australian opal and the way in which it attains its value.
What is Opal?
Opal is an amorphous form of silica that is deposited in very unique geological circumstances that occur rarely in the world. When it is deposited as regularly stacked and bonded microscopic spheres, white light is diffracted by the gaps between the spheres. As a result different spectral colours are reflected and seen. The more regular the stacking, the larger the pattern.
The value of opal depends largely on body colour, the play of colour, brightness, pattern, size and weight.
- Body Colour – this is the background colour on which the opal is seen. The colour ranges from white to grey, grey to black (most valuable) and almost colourless (*crystal opal) for most of the solid opals.Boulder opal generally has from a light tan to very dark brown background but can also be found with a white body colour to even black. When boulder is found with this black background it is termed “black boulder” or “boulder black” and is highly prized.
- Play of Colour – this is the term given to the movement of colours seen in the opal face. Colours range through all the spectral colours from purple through, blue, green, yellow, gold, orange, red, and magenta, these “move”, “roll” or “dance” across the gems surface as one or more colours. Purple is the most common colour and therefore the least expensive of the colours. The value increases through the spectral colours with red and magenta being the rarest of all. A top quality opal will display all the colours with red in predominance.
- Brightness – This is the intensity of the actual play of colour, the brighter the opal the greater the value.
- Pattern – The size and the shape of the colour patches in opal are classed as ‘pattern’. The larger and the most regular patterns are worth the most. There are many different patterns, these are just a few examples. “Pin fire” is the most common pattern and consists of many tiny dots of colour throughout the gem. ”Silk” or “mossy” patterns are also common patterns. “Flagstone” as the name suggests resembles a layout of a flagstone path, “Floral” pattern resembles a bunch of colourful flowers and a true “harlequin” resembles a patchwork quilt and is the rarest pattern of all.
- Size and Weight – The size and shape of an opal has a great bearing on the price. As with other gem stones the smaller opals are more common and therefore are less expensive under 1.00 carat. A good high domed oval gem is the industry’s highest standard. Opals between 3.00cts and 10cts are in greatest demand in relation to Lightning Ridge black opal, therefore the price paid is generally more per carat. Gems in excess of 20.00cts are mostly in demand in boulder opal. The origin of the opal as well as the size and weight has a bearing on the value. As an example a small oval Lightning Ridge black opal of about 7mm x 5mm to 8mm x 6mm may weigh around 1.00carat depending on the thickness of the opal as well as the height of the cabochon (dome). A similar size boulder opal may weigh slightly more due to the heavier backing. (This is taken into account when pricing).