What is an Opal?
The name Opal was probably derived from Sanskrit “upala”, meaning “valuable stone”. This was probably the root for the Greek term “Opallios”, or a “stone from several elements”. So, the ancient Romans may already have had an inkling why Opals show such a striking play of colours. Unlike most gems, Australian Opals are infinitely varied and are most valued for their “play of colour” … the shifting patterns of highlights that can range across the entire spectrum.
Opal is a rare type of hydrated silica its chemical formula is SiO₂·nH₂O. The minute particles of silica form into closely packed spherical aggregates. It is the varied arrangement of these minute particles and the gaps between them that causes light to be scattered and diffracted.
Today in the gem centres of the world, Opals are recognized as precious gemstones on a par with Diamonds, Emeralds, Rubies and Sapphires. Opals, like any commodity, are subject to market forces and the increasing rarity of quality solid stones continues to drive the price of Opals up. Black Opal is the rarest Opal and most prized Opal. Black Opal with a black or dark grey body shows the most brilliant play of colours imaginable. The Queensland Boulder Opals are also rare and beautiful, which many experts declare to be the brightest and most attractive of all the Opals found in Australia.
All of Nature’s splendour seems to be reflected in the manifold opulence of fine opals: fire and lightning, all the colours of the rainbow and the intense shine of deep seas. Australia is the classical country of region. Almost 95% of all fine opals come from the dry and remote outback deserts of Australia.
The Roman author, Pliny called Opal a gemstone which combines the best possible characteristics of the most beautiful of gemstones: the fine sparkle of Almandine, the shining purple of Amethyst, the golden yellow Topaz, and the deep blue of Sapphire, “so that all colours shine and sparkle together in a beautiful combination”.
Play of Colour in precious Opal
Opal’s colour play emanates a very special attraction and fascination. What causes this phenomenon? This question was impossible to answer for a very long time. Only when in 1960s, a team of Australian scientists analysed Opals with an electron microscope, it was discovered that small spheres from silica gel caused interference and refraction manifestations, which are responsible for that fantastic play of colours. The spheres, which are arranged in more or less compact structures, succeed in dissecting the light on its passage through the gemstone and turning it into all the colours of the rainbow, always new and always different.
Different types of Opal
Black Opal
Black Opal is a generic term given to any opal which has a dark body colour when viewed from above. “Play of Colour” within a dark or black body colour will enhance the brilliance of the colours. Some have a transparent crystal colour bar overlaying dark opal potch, giving the otherwise light opal a dark appearance. Even expensive black opals may have only a thin colour bar on black potch.
Black Crystal Opal
A Black Crystal Opal is a unique and beautiful variation of a Black Opal. Black Crystal Opal is scarce and so unique. The base of the Opal is black, but it is translucent.
Translucent Black Crystal Opals only allow some light to pass through and can have inclusions inside or a deep darker colour. Opaque Opals that do not allow light to pass through are Black Opals not Black Crystal Opals.
Boulder Opal
Boulder Opal is a variety of precious opal that forms naturally within the host rock, often just a thin vein of precious opal is present.
Fine grade boulder opals have the top surface covered with precious opal. Slight marks and protrusions of ironstone in the face of the stone are fairly typical, and can affect value. Boulder opal is usually cut to the contours of the opal vein creating a freeform wavy surface and they are often irregular in shape.
Boulder Matrix
Veins or patches of precious opal intimately mixed with the host rock are referred to as boulder matrix opal. This form is found mainly in Queensland. Matrix Opal increases in value with brilliance as the quantity of precious opal becomes a higher proportion of the face of the stone.
Boulder Splits
These are formed where a single piece of rough is split along the opal vein to form a matching pair.
Opal Nuts
These are commonly referred to as “Yowah Nuts” although they are mined at both Yowah and Koroit in southwestern Queensland. They derive their name from their nut-like appearance. The opal occurs in kernels inside an ironstone concretion.
White Opal
Light Opal is commonly referred to as “white”, although this term should be used only where the body colour is milky and white. Most of white opals are found in South Australia. Brilliant light opal can be extremely valuable.
White Crystal Opal
The translucence of a white crystal opal often gives it a greater clarity and vibrancy of colour than opaque stones. Pale coloured white crystal opals are generally more valuable than opaque white opals.
Doublet and Triplet Opal
A Doublet Opal is a layer of precious opal bonded to Black Backing, simulating black opal. Doublets are usually more expensive than triplets because more opal is used.
A triplet opal is a thin veneer of precious opal bonded to a black backing with a protective crystal dome glued to the top.