WORLD'S MOST VALUABLE GEM

Opal is an extremely precious gem. Gem Black, Boulder, Crystal and White Opals are the world's #1 most valuable commercially available gemstone yet they are complex with an exponentially large range of valuation characteristics than Diamonds, Rubies, Emeralds, Sapphires or Tanzanites.

You may have heard statistics being shown depicting tanzanite or other gemstones as being '1000 times rarer than diamond'; this is only suggested because there are 100 miles of tanzanite mines worldwide, and when compared to the 100,000 miles of diamond mines worldwide. However, as there is only 20 square miles of black Opal mining, black Opal is actually 5000 times rarer than diamond.

Opal is a diminishing resource and there is anecdotal evidence that suggests an increase of up to 25% per annum in boulder Opal prices and 15% per annum increases in white, crystal, and black Opal.

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HOW TO VALUE OPAL

If you would like to find out the value of your Opal, bring it to Australian Opal Cutters! We have extensive experience in cutting Opals for over 35 years, and use an industry standard program called 'Smart Chart'  to value any natural Opals with a detailed and systematic approach.

This "industry leading" program assesses the essential characteristics of an Opal to determine its value. These characteristics include:  Variety, Body Tones, Brightness, Transparency, Colour, Hue, Outline, Profile, Pattern, Display, Distribution, Inclusions and Weight.

We recommend having your Opals valued every few years as fine quality Opal will increase in value each year as much as 15%.

Written valuations are available for $125 for each loose Opal.

The Diamond industry has developed a robust valuation model for Diamonds. Most gem valuers have been trained to carefully assess the "4 C's" or the Cut, Colour, Clarity and Carat weight. Unfortunately these values are only 4 of the 14 categories that need to be assessed in order to finalize the value and (unfortunately) when you use a Diamond valuer to valuer an Opal the result is unreliable.

If you are looking to make a purchase, or investigating a family heirloom then these characteristics are a simplified method to give you some confidence. These would be the 3 most important factors if you are looking to make a purchase or get a basic idea of the value. Opal value is ultimately all about the colour, brightness and pattern.

COLOUR

 Is best when multi-directional. You should be able to see different colours from variety of angles. Spin the gemstone around to see how many colours in the spectrum you can see. As a general rule if you can see pink or aqua you will generally be able to see all of the colours of the spectrum at some point. It is important to note that Opal colour can show completely different spectral colour bands in different 'types' of light. So try looking at the Opal in daylight (sunlight), UV light, with a torch as you may see completely different colours revealed and this adds value!

BRIGHTNESS

Intense, brilliant colour is more valuable. High quality Opal will show you colour without direct light. We call this the 'miners test'.Simply take your Opal out of the bright showroom or sunlight and hold the Opal 'cupped' in your hand creating a shadow. Opals with `gem quality' colour will show intense flashes or display this colour even out of the direct light.

PATTERN

Patterns occur when the array of molecules are perfectly aligned in repeated sequences (blocks of molecules with the same structure). It is staggeringly rare and makes up only 0.01% of all Opals. Opals with ‘named’ patterns are far more valuable than regular Opals as they are so much rarer. This possibly accounts for the greatest confusion in the Opal industry as there may be two Opals side-by-side that look ‘basically the same’ (to a novice) yet one has a ‘named pattern’ which increases the value exponentially.Patterns such as ‘pinfire' , ‘chaff’, ‘straw’, ‘sheen’, ‘feather’ to name just a few and, when you find large blocks of colour you have the elusive ‘Harlequin’ pattern, the #1 most valuable commercially available gemstone in the world. To give you an idea of the rarity of this pattern we have cut and traded literally millions of Opals in over 60 years and have seen less than 10 genuine Harlequin patterned Opals. One Miners that we work with has been mining for 20 years and has never uncovered a harlequin patterned Opal.

Be cautious of anyone who tries to tell you what your Opal is worth without first assessing the 13 characteristics of the Opal. Opal is in the TOP 10 most valuable gemstones in the world, so be sure to ask your valuer: "How many characteristics do you look at when valuing an Opal?" or ask which valuation program your valuer has used to assess value, or which certification they have done to qualify them to value and Opal.

Most importantly, once your valuer has determined the value ask: do you have any comparable examples for this value? Where could I see them? This question is important as valuers have been known to ‘google’ the value of an Opal by simply entering in the characteristics into a search and then using the result to define a value.

The problem with this is in the integrity of the search results (or lack of). It is a huge problem online that many thousands of tens of Opals that are advertised as "Australian" or "Natural" Opal are actually synthetic Opal,(a complex plastic) or Ethiopian Opal (which is regularly dyed, heated and treated) which is far less valueable than Australian Opal but looks exactly the same.

If it is just an "opinion" of value and does not answer the above characteristics it is basically not worth considering.

So please contact Australian Opal Cutters to find out the TRUE value of your Opal!

NOMENCLATURE

To accurately define the value of an Opal it is important to work out the definitive characteristics that establish value. A small difference in (say) the body-tone (N scale) can have a big difference in the final value. Unfortunately there are numerous anecdotal instances where international valuers have called a "Solid Opal" a "Doublet Opal" and valued the gemstone accordingly (to the detriment of the owner).

The process that is used by Australian Opal Cutters to place value on Opal has been developed through many years using the Opal valuation software known as "Smart Chart" and uses principles that have been developed by Industry representatives (Gemmological Association of Australia GAA) and associations (The Opal Association) over past decades.

The use of standardised descriptions (or “Nomenclature” / descriptive naming system) for OpaI is considered a key in establishing the correct value for the various types of Opal. For example, when a customer buys a Black Opal at some expense they need to be confident that is indeed what is widely recognised as a “black” and not a lesser stone (White or crystal Opal).

In the example mentioned previously where International valuers have been known to confuse (or incorrectly value) Black or Boulder Opals mistaking them for Doublets this is due to the similarity of the Iron Oxide ‘bars” or Boulder Opal ironstone base (in a Boulder Opal). It is understandable as some top gem black opals actually have a flat face and can have a very straight line to differentiate the colour bar from the base tone and looks exactly like a doublet (but is not). It is understandable that diamond valuers could even confuse the actual “types” of Opal (which can affect value dramatically) as there are so many elements involved.

Over the past few years the Gemmological Association of Australia (GAA) has worked at developing a defining nomenclature. The Opal Association and the industry in general strongly endorses the use of these definitions with the aim of standardising nomenclature across the Opal industry worldwide.

How to best describe Opal has been a contentious and difficult issue for a very long time and may well remain so for some time to come. However, as a consequence of factors such as: growing international and local awareness of Opal as a major Australian resource; the emergence world-wide of a real desire to standardise all terminology related to gemstone and the ever growing number of synthetics and limitations that are appearing in world markets; it has become necessary to agree on some well based concepts of how a unique gem such as Opal, should be described and this has motivated the major influencers in the Australian Opal industry to work more closely together to agree on a uniform procedure and process to classify and Opal.

Today the Australian Gemstone Industry Council and The Gemmological Association of Australia (GAA), the Australian Gem Industry Association (AGIA), the Lightning Ridge Miners Association (LRMA), the Jewellers Association of Australia Ltd, and the Opal Association have all agreed-to, ratified and published the following standards for the Opal valuation process. The nomenclature and classification of Opal, that follows, is reproduced, verbatim, from the Resolutions of the Federal Council of the Gemmological Association of Australia.

Opal is Australia's National Gemstone. Australia produces 95% of the world's supply of precious Opal. This nomenclature encompasses all types and varieties of Opal. It provides a standardisation of terminology for the market

Opal is a gemstone consisting of hydrated amorphous silica with the chemical formula SiO2nH20. There are two basic forms of Opal described by visual appearance. "Precious Opal” is Opal which exhibits the phenomenon known as “play-of-colour”, which is produced by the diffraction of white light through a micro-structure of orderly arrayed spheres of silica. "Common Opal" (also known as "Potch") is Opal which does not exhibit a play-of-colour. The distinction between “common Opal” and “potch” is based on their formation and structure. Potch is structurally similar to precious Opal but has a disorderly arrangement of its silica spheres. Common Opal shows some degree of micro-crystalinity but as the molecules are scattered they do not form rainbows inside the Opal.

The nomenclature has not been designed to force any changes to the various colloquial terms used to describe Opal in Australia, or indeed in countries overseas such as Mexico. Colourful language, Australian colloquial terms for Opal, and terms that have been a part of the Australian scene for hundreds of years have added significantly to the mystique and folklore of everyday language used on the Opal mining fields. Expressive local terms and older historical terms always will exist in the Opal miner’s vocabulary. These will remain to have their rightful place in our

gemstone history and in the tale-telling for years to come. Terms such as “nobbies”, “semi-black”, “white-crystal”, “black-crystal”, “rubs”, “fire”, “solid”, “enhanced”, “treated”, “natural” are not referenced in the categorisation.

The purpose of the nomenclature, therefore, remains to provide a basic, yet “more official” description of the gemstone we all prize and know as Opal. This nomenclature is for everyone to use and understand. Simple descriptive terms, that can be used by the majority of people, from the customer to the scientist, have been chosen. These provide the gemstone industry as a whole with a logical and unbiased way of grading and evaluating Opal. However, simple terms do become difficult when the many different types, formations, pseudomorphic fossil replacements, mineralogical types, and geological occurrences of Australian Opal are considered.

This nomenclature aims to remove from common usage the following terms that have caused linguistic problems and confusion. These terms are ‘semi-black’, ‘grey’, and ‘solid’.

To begin with the first part of the nomenclature, mention is made of precious Opal, potch and common Opal. The best way of determining the difference between these is to observe whether or not the Opal you are viewing shows the phenomenon which we all know as play-of-colour.

It is possession of this optical phenomenon for which Opal is most prized. The differentiation between these basic forms of Opal is therefore quite simple. If the Opal displays a play-of-colour it is termed precious Opal. If a play-of-colour is not displayed, then the Opal is either common or potch Opal. While it is recognised that the term precious is neither a scientific nor gemmological term, it is retained in this nomenclature for simplicity, and with the intention of further enhancing the value of Opal as a gemstone by removing it from any historical association with ‘semi-precious’ gemstones.

In an attempt at keeping the nomenclature simple to use, the terms common Opal and potch Opal have not been separated. It must be recognised, however, that there are distinct mineralogical differences between potch and common Opal. (Jones & Segnit, 1971).

The term ‘solid’ has been removed from Opal terminology, for the simple reason that all types of Opal are essentially solid from a scientific point of view. That is, Opal does not exist naturally either as a liquid or a gas. ‘Solid’ has been replaced by the gemmological term natural Opal. Correlating with this use is the recommendation that when describing doublets and triplets that the term composite be used instead of ‘assembled’.This also is the terminology currently recommended by CIBJO.

Essentially there are three types or forms of natural Opal, which are termed simply Opal, boulder Opal and matrix Opal. Perhaps the most contentious issue in the

nomenclature concerned introduction of the term body tone, to describe the comparative lightness or darkness of an Opal as distinct from its play-of-colour. Technically, it would have been best just to have two types of ‘body tone’ — either ‘black or white’ or just ‘light or dark’. However, the sub-committee rightly decided not to attempt to change too much of the terminology that had been in common use for over a hundred years.

So, inclusion of the term black Opal was considered to be an imperative. Following much discussion the term body tone was included in the nomenclature to describe the comparative lightness or darkness of Opal — irrespective of its play-of-colour. The term tone, which is used by colour science, is in agreement with terminology used internationally to describe the lightness or darkness of particular hues or colours.

This nomenclature for Opal has been designed for use throughout the gemstone and jewellery industry, not only in Australia but internationally. While preparing this nomenclature, the sub-committee has been cognisant of conventions of international trade organisations, such as the International Confederation of Jewellery, Silverware, diamonds, pearls and stones (CIBJO), the International Coloured Gemstone Association (ICA), as well as the linguistic problems associated with different languages and the differing connotations these languages may place on an internationally acceptable nomenclature.

VARIETY

The “Variety” of an Opal is determined by where the Opal Body Tone is located within the Body Tone Chart

Opal that has a body tone of N1-N4 is defined as “Black Opal”. Opals mined in the “Mintabie” fields in South Australia have been known to produce excellent quality Black Opal however are more famous for “Dark Opal” or “semi-Black” or “Grey” Opal, terms used to describe Opals with body tones lighter than N4.

Whether an Opal is a black or not can be determined by merely examining the face of the stone. All colours are ignored and the overall body tone (blackness level) is estimated. This is compared to the scale of blackness shown below stones achieving values of N1 to N4 on this scale are considered Black Opals – attracting the additional value associated of this class of Opal.

White Opal or “Light” Opal is “Light” or white in appearance this is the most abundant and affordable form of Opal. Opal is hydrated silicon dioxide, which

means it has a water content and an amorphous crystal structure. White Opal is the material that is also used for Doublets or Triplets which have spread in international popularity due to their brilliance and affordability.

Crystal Opal is pure hydrated silica it is translucent so you can see straight through it. We prefer the colour to be multi-directional, so you can see colour from all different angles, the second thing we prefer is large blocks of colour and the third thing is strong play of colour that shows up in the dark.

Natural Opal is Opal which has not been treated or enhanced in any way other than by cutting and polishing. There are three types of natural Opal. The “Type” of Opal is determined by analysing the Opal is “Solid” hydrated Silica or if it is dispersed with an ironstone host.

NATURAL OPAL TYPE 1

Is Opal presented in one piece in its natural state apart from cutting or polishing, and is of substantially homogenous chemical composition.

black-opal
Bolder Opal
N1,N2,N3,N4
Opaque
dark-opal
Dark opal
N5,N6
opaque/translucent
Light-opal
Light Opal
N7,N8,N9
opaque to transparent

NATURAL OPAL TYPE 2

Is Opal presented in one piece where the Opal is naturally attached to the host rock in which it was formed and the host rock is of a different chemical composition. This Opal is commonly known as boulder Opal

black-opal
Boulder Opal
N1 to N9
opaque/translucent

NATURAL OPAL TYPE 3

Is Opal presented in one piece where the Opal is intimately diffused as infillings of pores or holes or between grains of the host rock in which it was formed. This Opal is commonly known as matrix Opal.

black-opal
Boulder Opal
N1 to N9
opaque/translucent

NATURAL BLACK OPAL TYPE 1

The most valuable and popular of all Opals is black Opal. Black Opal is incredibly rare and is found at Lighting Ridge in Northern NSW, so called because in a terrible Lightning storm a farmer, his dog and 600 sheep were all killed by Lightning. It is called Black Opal because it has a black base caused by black or grey iron oxide in the Opal. This black potch or common Opal has no value unless we find a colour bar on top of it. The color bar or the ‘play of colour’ of Black Opal comes in all the colours of the rainbow with red being the rarest and most expensive and may be designated N1, N2, N3 or N4 on the Scale of Body Tone.

Black Opal has a blue-black to light charcoal body tone and is vary rarely found outside Lightning Ridge in New South Wales. The dark background serves to highlight the colour-play of dramatic spectral flashes. Fine examples of this variety are the most expensive per carat and rival rare pink and red diamonds in price. Black Opal is found as what the miners call ‘Nobbies’, these are fossil replacements of corals or sponges. During its formation, the replacement of organic material by Silica resulted in carbonaceous material or impurities like titanium impregnating the mineral structure giving Black Opal its body colour.

black-opal

Below are Black, dark and light precious Opals displaying a strong play-of-colour.

black-opal

Black Opal of N2 body tone and a dominant red-orange play-of-colour

black-opal

Black Opal of N3 body tone and a dominant blue-green play-of-colour

NATURAL DARK OPAL TYPE 1

Is the family of Opal which shows a play-of-colour within or on a dark body tone, when viewed face-up, and may be designated N5 or N6 using the Scale of Body Tone.

Dark Opal has a smoky to dark grey body tone and is mostly found at Lightning Ridge NSW however the Mintabie field in South Australia is famous for Dark Opal and Dark Crystal Opal.

Black Opal hasa blue-black to light charcoal body:one and isvary rarelyfound outside Lightning Ridge in New South Wales. The dark background servesth highlight the colour-play of dramatic spectral flashes. Fine examples of this variety are the most expensive percarat and rival rare pinkand red diamonds in price. Black Opal is found as what the ml ners ca II ',lobbies., these are fossil replacementsof coralsor sponges. During its formation,the replacementof organic material by Silica resulted in carbonaceous material or impurities like titanium impregnating the mineral structure giving Black Opal its body colour.

black-opal
Semi-Black, Dark Crystal, Grey, Dark Grey, Smokey Crystal, Smokey Jelly.
black-opal
Dark Opal of N5 body tone and a green play-of-colour.

NATURAL LIGHT OPAL TYPE 1

Is the family of Opal which shows a play-of-colour within or on a light body tone, when viewed face-up, and may be designated N7, N8, or N9 on the Scale of Body Tone. The N9 category is referred to as white Opal.

Coober Pedy was discovered in 1915. This is where most of the ‘white’ or ‘milky’ and crystal Opals (together known as a ‘light Opal’) are mined. Coober Pedy is the main producer of white precious Opal, which is predominantly seen in stores overseas, particularly in the USA. Today, the Opal fields encompass an area of approximately 45 square kilometers. The Opal level is formed of soft pinkish clay mixed with soft bleached sandstone.

The name “Coober Pedy” is an Aboriginal word that translates “man in a hole” and with temperatures around 40 degrees Celsius or 100 degrees Fahrenheit all year

the cool underground miners have become popular as living quarters, and now most of the locals live underground!

Opal with a distinctly coloured body (such as yellow, orange, red or brown) should be classified as black, dark or light Opal, by reference to the Scale of Body Tone, and also have a notation stating its distinctive hue appended to its determined body tone.Light Opal has a body tone ranging from milky white to transparent. Light Opal is the most common variety found at most fields but mainly at Coober Pedy and in South Australia. White Opal gives the full colour array on an opaque background, whereas Crystal Opal is transparent to translucent without milkiness and has bright colour flashes suspended in its midst. Crystal is superior to white, grey and jelly. Jelly Opal exhibits a moderate play of colour within a transparent background.

black-opal
Boulder Opel: Displaying green chaff and pinfire patterns
black-opal
Variety: White, Crystal, Jelly Opal N3 body tone and a red-blue play-of-colour in a ‘rolling flash’ pattern.

NATURAL BOULDER OPAL TYPE 2

The boulder Opal is one of the rarest and most valuable forms of Opal found in Australia and makes up less than 5 % of all Opal mined. It is very sparsely distributed through South West Queensland. It is predicted that boulder Opal is going to run out in the next 10 years because of the difficulty clearing Native Title and EPA requirements of rehabilitation. Native Title is the process of gaining agreement from the local Aboriginal tribes before mining takes place. This is an extremely difficult and time consuming process. Added to this is the EPA requirement that all mines need to be “back-filled” and trees planted when the mining is finished. This process alone can send a miner bankrupt if he has not found “colour” in his “dig”.

Added to this are the onerous paperwork requirements, with “enough forms to sink a ship” the average miner just does not have the motivation to comply with

all the government regulations. So, as a result there are fewer and fewer miners on the field! Boulder Opal is formed in the cracks and crevices of the ironstone boulders in a gel form possibly as recently as hundreds of years ago, and with the passing of centuries this jelly Opal turned solid and as you are left with some beautiful boulder Opal specimens. Boulder Opal occurs as a filling between the concentric layers or in random crevices in the ironstone. The cutting process is extremely difficult as the cutter must navigate the “hills and valleys” of the Boulder Opal surface. What we are left with is an incredibly unique and individual gemstone.

Boulder Opal has a high loss factor when cutting as we only yield 5 % and has a rock waste factor of 95 %. It is also the only Opal suggested to run out within the next 5 to 10 years and with the value (“it is suggested”) increasing by 15-25 % each year the Boulder Opal can certainly make a sound long term investment.

black-opal
black-opal

BODY TONE

The body tone of an Opal is different to the play-of-colour displayed by precious Opal. Body tone refers to the relative darkness or lightness of the Opal, while ignoring its play-of-colour.

To determine the body tone of an Opal one examines the piece of Opal, “face-up”, and determines (by visual comparison) its position in the scale of body tone.

If the tone of the Opal appears darker than N4, then the Opal may be classified a black Opal. Consequently, any Opal with a body tone darker than N4, irrespective of hue, can correctly be termed black Opal. Some boulder Opal possesses this body tone, so it is very correctly termed black boulder Opal. It is also appreciated that some very dark red Mexican-type Opals would have dark enough body tones to be categorised as black Opal.

If, on the other hand, the tone of the Opal corresponds to N7, or lighter, it is classified as light Opal. If this light Opal also has a hue, then it is termed, for example, a light yellow Opal.

The boxes (below) comprising this scale, represent approximate values of body tone in equal intervals from black to white. This arrangement is in agreement with all known scales of tone used in colour science, and is well illustrated in the commercially available Rock-colour Chart produced by Geological Societies in America.

The Scale of Body Tone, as illustrated in the nomenclature, ranges from N1 to N9. The prefix “N” reflects the neutral tone of this scale. The steps in the scale of body tone, which are arranged to indicate approximately equal decreases of darkness.

After examining current industry standards, the N4 category was decided to be the cut-off point for black Opal. The AGIA is currently attempting to produce a scale of body tone, using commercially available computer scanning devices and suitable software. However this scale is not yet available.

If the Opal is lighter than N4, and its tone corresponds to N5 or N6 on the scale of body tone, then it is classified as dark Opal. If, in addition, this Opal has a decided hue colour, it is additionally classified as, for example, a dark blue Opal.

The current reference, used by the Lightning Ridge Miners Association, is the neutral tone scale specified in the American Geological Society’s Rock-colour chart. This has proved to be a good guide, for in most instances it will be possible to correlate the different ‘tone scales’ into a simple and repeatable system. An acceptable descriptive term was sought also to describe those Opals that have distinct body colours or hues, such as those displayed by both Mexican fire Opal and honey Opal from Lightning Ridge — considerable amounts of which consists of common or potch Opal. However, as an acceptable all round term could not be found to describe these Opals, the committee decided to describe them by determining their body tone/s, their primary and secondary body colour/s or hues, and their transparency.

BRIGHTNESS

Vividness of colours is of paramount importance – the brightness of an Opal is directly related to price. It is not uncommon to observe several levels of brightness in the face and variation in a stone's body tone. An assessment is made on the overall impression of a stone. One of three categories can be selected:

30% = Subtle, 30-70% = Bright, 70%(+) = Brilliant.

TRANSPARENCY

Opal shows all forms of diaphaneity that range from transparent to opaque. Natural precious Opal which is transparent to semi-transparent is known as crystal Opal. Crystal Opal can have either a black, dark or light body tone. In this context, the term ‘crystal’ refers to the appearance of the Opal and not its crystalline structure.

Light Opal of the 'Crystal' variety is found on most Opal fields and is transparent. A dot placed on the back of a stone using a black felt pen should be clearly visible from the face. Black Crystal may be transparent to translucent when held up to the light however it is exempted from the above test due to body tone.

The majority of solid Opals are translucent, transmitting and diffusing light so that objects beyond cannot be clearly seen. Crystal Opal from South Australia, particularly in red stones, may possess superior hues of rich red. However the clarity and purity of Andamooka Crystal, coupled with brilliance and relative rarity, make it the more valuable stone quality for quality.

Most Lightning Ridge Crystal is cut into a pleasing high dome which enhances the depth of pattern. A transparent Jelly Opal or Crystal Opal will usually be more desirable than an opaque White 'Milky' Opal. Conversely Black Opals with an N1 or N2 body tone are opaque and may fetch the highest prices achieved per carat.

When to term an Opal a crystal Opal also provided considerable discussion. The key to classification as crystal Opal is really the transparency of the Opal. Perhaps a better term would have been ‘transparent Opal’; but any change in terminology from crystal to ‘transparent’ may take many years to progress. The obvious problem with the term crystal Opal is, of course, the basic fact that the Opal has no crystal structure. Again the sub-committee decided that it was unwise to change a term that had been in common use for so many years. The sub-committee further believes that overseas gemmological communities may yet force a change in this usage, if strict terminology is ever to be implemented.

The range of transparency considered acceptable for defining crystal Opal (transparent to semi-transparent) was taken straight from Robert Webster’s discussion on transparency in his world-renowned textbook Gems. The committee decided that transparency did not need to be re-defined in the nomenclature; but just stated as a classifying category.

To grade the transparency of an Opal with the nomenclature, how transparent the Opal is must be determined. If the Opal is only translucent, then it is not termed crystal Opal. It should be remembered that in some instances the play-of-colour of crystal Opal will be so strong or brilliant that assessment of transparency, by the normal ‘read-through’ criterion, may not be possible as the Opal cannot be ‘read-through’. When this occurs the best test of transparency would be to ‘look-through’ the Opal with transmitted light. If transparency exists then this will be readily apparent. If the material remains only translucent, then it is correctly labelled as light Opal. It is hoped that future scientific advances may yield a better and more accurate method of assessing transparency.

A note also should be made concerning the removal of the term ‘jelly’ Opal. The basic facts are that due to the extreme transparency of this Opal it becomes a type of lower quality crystal Opal that displays subdued low quality play-of-colour. In spite of any restriction applied by this terminology the term ‘jelly’ Opal will probably remain in colloquial use for many years to come.

The description of composite stones requires only a small change in nomenclature. Instead of these Opals being described as ‘Opal doublets’ or ‘Opal triplets’, the nomenclature emphasises their composite nature by terming these doublet Opals and triplet Opals. In this terminology, which emphasises the composite nature of these Opals, it is the first word of the term that precisely identifies the material.

The rest of the nomenclature discusses Opal treatments, synthetics and imitations. These are not associated with the descriptive nomenclature for natural Opals, but have been included to complete the nomenclature. These descriptions are in accordance with the latest edition of CIBJO’s Classification of materials and Rules of application for diamonds, gemstones, and pearls.

COLOUR

“Colour” relates to colour “display” and how the hues are arranged on the face of a stone. When rotated through 360° in 90° segments most Opals will show a marked difference at each turn. Many showing a good play of colour at one angle, can be nearly blank from one or two other angles and a price penalty is imposed

dependent on the severity of the characteristic. 'Broad' or 'Flash' pattern stones often display this characteristic. Most stones look best in one particular orientation, some need to be tilted to be appreciated whereas the finest Opals are non-directional.

HUE

Refers to the dominance of hue in the fire colour. Rich saturated hues; emerald green, rich orange etc. are always present in the finest gem quality stones yet even pastel tones if bright can make a top quality stone. Conversely subdued

brightness and pale pastel hues can only indicate, at best, a commercial quality stone. The finest Opals hold their beauty in all types of light.

OUTLINE

Most often dictated by the rough form. Most Opals, particularly Black Opals, tend to be fashioned as ovals and because Opal is cut ‘en cabochon’ (with a domed surface) these features have traditionally been preferred for jewellery aesthetics and calibration purposes.

However, most Boulder Opals are cut as free shapes which can lend themselves to more distinctive designs. In the last decade there has been a strong trend towards cutting freeform 3-dimensional shapes from most gem quality Opal. By sculpting the rough, yield may be maximised in terms of weight and spread, aesthetic talent must be applied to balance the stone's lapidary design.

PROFILE

A stone with a domed surface will be more valuable than one with a flat or undulated surface. This is because the domed stone has more depth from which to emit play-of-colour. The ratio of the colour bar with play-of-colour to the thickness of potch or ironstone backing should be balanced and have a suitable setting edge for jewellery manufacture.

Poor cutting and polishing will significantly reduce a stone's value. Some stones have been cut disproportionately, a stone may have been left too thick (heavy on the potch or ironstone backside) relative to its spread or face area. Consideration is made of a stone's proportion and aesthetic balance when determining the absolute value per carat.

FIELD OF ORIGIN

Any indication of the “origin” of an Opal (by the use of geographical location), is not used unless it is qualified as an indication of the type of locality as recommended by the International Confederation of Jewellery, Silverware, Diamonds, Pearls and Stones (CIBJO) e.g. “Lightning Ridge type black Opal”.

The Lightning Ridge fields in far north NSW is the world’s only consistently mined source of Black Opal. Black Opal has a black or dark grey appearance when viewed from the front. The definition of the ‘tone’ of an Opal as being viewed from the front is due to the variances in colour bars that can occur throughout an Opal with the back of an Opal often presenting a different ‘body tone’ to the ‘face’ of the Opal. The black base tone is caused by black or gray iron oxide contained within the molecular structures within the Opal. Black Opal “potch” (or common Opal) has no value unless a colour bar is present and is often used as the backing material for Triplets (Composite Opals with three layers).

Coober Pedy was discovered in 1915. This is where most of the ‘white’ or ‘milky’ and crystal Opals (together known as a ‘light Opal’) are mined. Coober Pedy is the main producer of white precious Opal, which is predominantly seen in stores overseas, particularly in the USA. Today, the Opal fields encompass an area of approximately 45 kilometers. The Opal level is formed of soft pinkish clay mixed with soft bleached sandstone.

Australian Opal mining fields are::

Adavale, Andamooka, Beechworth, Coober Pedy, Coocoran, Grawin, Jundah, Koroit, Kynuna, Lambina, Mintabie, Opalton, Queensland, Quilpie, Sheep Yard, South Australia, Tintenbar, White Cliffs, Winton, Yaraka and Yowah. Lightning Ridge has achieved the status as the ‘world’s leading field’ for value.

COLOUR PATTERN

Patterns in Opal are very important. There are numerous “named” patters; pinfire, broad flash, harlequin, rolling mackerel just to name a few. This is where the molecular structure is so perfect it provides a repetitive display creating a “pattern”. These “patters” are very rare and something to look for when you purchase on Opal.

Almost as important and when combined with brilliance may increase price manyfold.

Generally a larger pattern is more valuable than a smaller pattern e.g. 'Harlequin’ is the most highly prized pattern whereas ‘Pinfire’ is a more common pattern. The descriptions may be termed otherwise, e.g. 'Broadflash' may be called 'Peacock' pattern, and the 'Cat's Eye' phenomenon is usually referred to as 'Rolling Flash' by the Opal trade.

A vivid pattern is more valuable than a static less playful one. A lively stone possess depth or saturation of colour and pattern. While every Opal has a unique pattern, there are seven categories of patterns that all Opals fit within: Pinfire, Flash, Broad Flash, Rolling Flash, Harlequin, Rare Patterns and Picture Stones. Over 90% of cut stones have either Flash or Broad Flash patterns.

Examples of the rare and elusive harlequin pattern

The “Rolling Flash” Pattern can be seen as an element in many of the others. It is where the colour ‘rolls’ across the surface or blocks of colour. IN striking examples this appears as a “full-faced flash” and is brilliant and impacts the eye with a ‘flash’ of moving colour. In lesser examples the ‘roll’ can appear seemingly ‘beneath’ the other patterns or spread across one element of the colour bars.

"Chaff" pattern is where small ‘lines’ appear that reflect small pieces of ‘chaff’ that have been separated when wheat is ‘threshed’ during harvest. These small lines can appear in parallel form or in a random array. The lines can display a variety of colour effects from rolling flash, to ‘sheen’ or sometimes can create an undulating colour effect as the bars break up or create their own pattern of colour.

A “mixed” pattern is where you have a number of elements from multiple patterns all working together. ‘Mixed’ patterns will often combine ‘Pinfire’ and ‘Chaff’ or ‘Floral’ patterns so that no one pattern is predominant yet elements of each are recognisable.

The ’Feather’ pattern is very rare. Like its namesake this pattern literally reflects the same lines, ‘sheen’ and play of colour that you would see on an actual feather. The elements of this pattern can be broken up into small (or large) elements that can appear as ‘fragments’ of the pattern or work together to cover the surface of the gem with a complete ‘feather’ (a full-faced feather is very rare and would add value to the gem placing it into a ‘named-Gem’ category.

”Floral” pattern has been described as a ‘blocky-yet-irregular’ array of colour fields that interlock in a style that reflects a uniformity. In extreme examples the colours are so regular that they could be described as a form of ‘harlequin’ pattern yet there are many iterations of this pattern and the ‘classic’ floral does look somewhat like small ‘blocky’ flowers.

The pinfire pattern is a regular array of pinspots of colour. These microscopic displays are a miracle as the molecular perfection that is required to form this repeated pattern is indescribable.This microscopic pattern can appear in isolated ‘pockets’ within a gemstone or in a variety of areas within a gem. Patterns such as “rolling flash’ can flow across the gem and create a ‘galaxy’ of colour and beauty that is absolutely unique.

The “Sheen’ pattern is where a semi-metallic reflection or irridescence appears in the colours. This pattern is very rare occurring more frequently in Opals from the Mintabie region. The reflection, brightness and intensity of a ‘sheen’ pattern can be quite striking and this is a genuinely collectible pattern.

Similar to the ‘Harlequin’ pattern in rarity and value the ‘Pallete’ pattern is where ‘Harlequinesque’ blocks of colour form (yet in a more irregular array). Gems that display ‘Harlequin’ or ‘Pallete-Harlequin’ patterns are the rarest commercially available gemstones you can purchase.

The ‘Ribbon’ pattern displays repeated flowing rolling bands of ‘rolling-flash’ that have been segmented into ‘Ribbon’-like bands. This parrtern can occur in isolation and is displayed when a ‘Rolling-Flash’ is repeated across a band of colour (rather than one rolling colour the colours repeat). Rare and stunningly beautiful this is a truly captivating pattern.

DISPLAY

‘Play of Colour’ is a unique visual phenomenon which sets precious Opal apart from all other gemstones. Also known as 'Fire'; An Opal may display one or more, and sometimes all of the spectral colours. These colours are seen within the grains of a pattern. As the stone is viewed from different directions, rotated and tilted, the colours of each grain may change or disappear.

The dominant hues in an Opal are referred to with the post script ‘-fire’ e.g. green-fire or multi-fire (multi-coloured). The spectral colours (red, orange, yellow, green, blue, indigo, violet - in order of value) are usually combined in varying amounts in most Opals. The predominant hue is listed first with the main secondary hue following, e.g. green-blue. The predominant hue should be at least 50% to 70% of the total fire (play-of-colour), the second or supporting hue should be around 20% or more.

Some combinations are far more attractive than others, e.g. red-blue or green-blue when hues are intense, while red-orange and green-multi-fire are not as desirable. Gem quality stones with a predominance of red or orange are enhanced by a complimentary touch of vivid blue.

Red Opal is the rarest and most valuable and a predominantly red stone may potentially display all of the spectral colours, if so it may be referred to as ‘multi-red’. Bearing in mind that absolute value depends on brightness, pattern and body tone; red and multi-coloured Opals are rarer than green-orange, blue-green and blue Opals in that order. Given all other factors are equal an Opal containing red can be valued greater than a blue-green-yellow Opal by a factor of up to 3 times.

DISTRIBUTION

Opal quality is markedly affected by the ‘depth’ of pattern and amount of 'fire' showing on the face. This factor is judged from the stone’s best angle.

Very few, with the exception of 'Flash' pattern stones, will show 100% of the face covered in colour. Consequently a sparse ‘distribution’ of colour attracts a lesser value.

INCLUSION

Not uncommon in the back of stones, generally these are small sand-spots and do not affect price drastically. However marks or cracks that are noticeable in the face of the stone will have a marked effect on the price of an Opal. Visible inclusions may include; patches or lines of potch, 'webbing', 'sand spots', crystals of gypsum and ironstone in the face of Boulder Opal.

'Windows' in Black or Boulder Opal where there is an area of transparency in an opaque Opal's body that allows light to enter through the back of the stone and so dilute its play-of-colour are detrimental to price. Certain types of sand and other inclusions are indicators of origin.

WEIGHT

Prices per carat are generally at their greatest for exceptional stones between 3 and 5 carats and up to about 10 carats, after which quality Opals in larger sizes become extremely rare and commensurately more valuable.

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