What is Opal
Opal contains every colour of the spectrum, from deepest and clearest blues and iridescent greens, through to golden orange, red and fuchsia; it can be pale and delicate, or dark and brilliant.
Due to its structure, opal may contain any combination of an infinite number of patterns and may reveal itself all at once - glorious from all directions – or it may be quiet and surprising, showing its greatest brilliance only during flashes of movement.
Indeed, precious opal is like no other gemstone because it changes colour as the observer turns the stone.
How is Opal formed?
The uniqueness of the opal extends further than just the physical stone that miners toil for and magnificent stones you see on display in opal shops and the stunning pieces created by specialist jewellery artisans. The creation of the opal is so unique that requires a special series of specific geological, climatic and biological elements to coincide to enable the formation of this elusive gem, and as such, it is only found in a few places around the world. Australia is fortunate to be one of those places and our central desert regions account for around 90% of the international opal supply.
These phenomena that contribute to the formation of opal extends back to the Cretaceous period (145-65 million years ago) and during that period the interior of Australia was made up of an inland sea and towards the end of this era, the water recede, refilled, and receded several times and continued to do so for may aeons, resulting in deposits of fine marine sands rich in silica being deposited on the ever-changing shoreline. About 30 million years ago, extensive weathering acting upon the stratified sediment and released large quantities of soluble silica.
Voids and cracks in the ground enabled the silica-rich solution to flow down and these are where the opal forms. Additionally, as organic matter of plants and animals was laid down into the sediment during the Cretaceous period and that in turn decomposed, the solution also filled these spaces to form the much prized fossilised opal. This process is very slow and is estimated by experts to take as long as 5 million years to form a 1cm thick opal vein.
The resultant gemstone is a non-crystalline silica, similar to quartz, but is not a mineral. Its internal structure enables unique diffraction of light to produce white, grey, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black.
Due to the unique nature of its formation, there are no steadfast rules of reasons how and where it is formed and as such, Opal mining is not an exact science. Many people, especially miners, equate opal mining as playing the lottery, but with a lot of digging.
Types of Opal?
Opal is known for its colour and is distinguished from other gems due to its ‘play of light’ or ‘fire’. This refers to the opal’s ability to show a vast array or all the colours the rainbow and are visible in the grains of the stone. When viewed, if the stone is tilted or rotated, the colours may change of disappear due to the change in direction of the light hitting the grain/s. This is what makes the opal so unique and strikingly beautiful.
Opals, those untreated other than than cutting and polishing are referred to as natural, while treated/modified Opals are refered to as 'man-made'.
Natural Opals are made up of:
- Natural – Light, Dark and Black
- Natural – Boulder
- Natural – Matrix
Man-Made Opals include:
- Man-Made - Treated Matrix
- Man-Made - Composites
- Man-Made - Synthetic
Opal classification is determined by ‘base/body tone’ and ‘transparency. Body tone refers to the stones tone which can range from colourless, white, and grey to black, while transparency refers to whether it is opaque, translucent or transparent. The clearer the transparency, the sharper the colour and these are often referred to as Crystal Opal.
Natural - Light Opals
Light Opals are those that exhibit colourless to medium grey in terms of base/body tone with many people classifying them as white (this should only be used if the specimen is in fact 'milky'). The Light Opal, which makes up the bulk of the precious opal are sourced primarily from Coober Pedy and Mintabie in the north of South Australia and to a less extent in White Cliffs (Outback NSW) where they were first discovered.
Natural - Black/Dark Opals
The most valued of all opals is the Black Opal, of which Lightning Ridge is ’home’ and their inherent value comes from their rarity. Black Opal is distinguished from other opals by their dark background (Body Tone) and this characteristic enables the brightness of colour of which they are known. The ‘darkness’ is the result of the opal is formed on a darker (black) quartz-like layer that enables greater refraction/reflection of the light to the top of the opal, especially the reds and pinks. It is the 'reds' that are more valuable.
To expand the 'play of color' of Black Opal even further, some specimens have a light crystal colour bar on dark opal potch (colourless opal) which gives the otherwise light opal a dark appearance. Even expensive black/dark opals may have only a very thin colour bar on black potch (Colourless Opal).
Black Opal is found as what is referred to as 'Nobbies', which are fossil replacements of corals or sponges. As the opal is formed, silica replaces the organic material and carbonaceous material or impurities like titanium impregnates the mineral structure giving the Black Opal its body colour.
Compared to Light and Boulder opals, Black Opals fetch a higher price for a given colour, clarity and pattern, due to their scarcity.
Natural - Boulder Opals
Formed in areas of Ironstone, Boulder Opals are created when the host rock (Ironstone) forms with/as part of the opal when the gem is formed in cracks/voids within the host rock. The resulting opal is often a thin vein laid on (natural) the Ironstone.
Boulder Opal, with colours ranging from black or light, is sourced from areas in Western Queensland, which are cut to incorporate the brown host Ironstone and are valuable due to their high demand. Around Andamooka in South Australia, where the host-rock is quartz which is lighter in colour, the Boulder Opal is also referred to as a ‘Painted Ladies’.
Natural - Matrix Opals
Matrix Opals are silicified sandstone or ironstone which has opal forming within infillings of pores or holes or between grains of the host rock.
Queensland Boulder Matrix Opal is distinguished by its ironstone host rock, while Andamooka Matrix Opal is a porous material from Andamooka, South Australia, which is often treated to enhance the colour by depositing black carbon via chemical treatment in the pore spaces in the stone. The Andamooka Matrix Opal is classified more as a made-made opal.
Man Made - Composites:
Composites a slice of Light Opal and adhered to a back of black potch, plastic or boulder. While they are popular as souvenirs, they are not investment quality stones and lack the durability of the natural variety with the veneer of the opal and back vulnerable to splitting due to moisture and heat.
Composites can be triplets or doublets, with the former being a veneer of opal on a black backing, while the triplet is a doublet with protective layer glued to the top. Doublets, with generally more opal used in their construction, are generally more expensive than the triplet.
Getting your Own Opal
Time in Lightning Ridge will inevitably turn to buying opal and the Ridge has plenty of opal shops and opal jewerly artisans. Check out our Buying Opal section for more information
If the partner is wanting but the wallet is unwilling, you can always fossick for your own opal at the visitor centre where a regularly replenished supply of material is provided and many who try their luck have been pleasantly surprised. In fact, last year a couple found a piece of Black Opal valued at over $20,000. So it can certainly be worth it. ** A note of caution though, this is the only place in town that you can do this and fossicking in someone else's claim, or 'ratting' as it is known, is not something that is taken too kindly and should be avoided.