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) potch 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 (Shaded rather than coloured opal) which gives the otherwise light opal a more intense appearance. Even expensive black/dark opals may have only a very thin colour bar on black potch.
Compared to Light and Boulder opals, Black Opals fetch a higher price for a given colour, clarity and pattern, due to their scarcity.
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.
The formation of opal goes back to the Cretaceous period (145-65 million years ago). During that period the interior of Australia was made up of an inland sea resulting in deposits of fine marine sands rich in silica being deposited on the ever-changing shoreline. About 30 million years ago, extensive weathering 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, plants and animals laid down in the sediment during the Cretaceous period decomposed, silica filled these spaces to form the much prized fossilised opal. This process is very slow.
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.
'Composites' are a slice of Light Opal and adhered to a back of black potch, plastic, glass 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.