Black Opal Brightness and Body Tone Guide

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The Opal Body Tone Guide was designed and created by the Opal Association of Australia and acts as a guide for Black Opal and Boulder Opals. The Opal industry has recently also been using this guide for many Opal types including Ethiopian Opals.

Body Tone N1 - N4 - Is for black tone body types.

Body Tone N5 - N7 - Is for light coloured Opals mined also at Lightning Ridge but these lower body tone grades can and have been used on Coober Pedy Opals and Ethiopian Opals.

Opal Brightness Guide

The Opal Brightness Guide assists in assigning a value to how bright an Opal is to the naked eye.

Opal brightness can be described as

Vivid
Very Bright
Bright
Moderate
Subdued

How is Opal Color Produced? Opal formation

The local name given potch, such as desert sandstone, is dependent upon the area in which they occur, but they all belong to the same families of felspathic sandstone, claystone, siltstone and mudstone, which are said to have been deposited over large areas buy streams and lakes.

We call any material that opal colour has formed on as Potch. Except for volcanic opal, which usually occurs in Tertiary rocks, all the types of opal we have come to know so well are found in these strata. Although the opal is found in the Cretaceous sediments, it is a matter of conjecture as to it’s deposition over millions of years during the Tertiary period. The general teaching is that certain minerals were broken down in the country rock to produce silica at 120 parts per million, and trace elements which were deposited in cavities left by bones, shells and other dissoluble material. These, as well as faults and fractures in the ground, provided pathways for the underground waters to carry the silica down at the end of the weathering period.

Although this is the entrenched theory, with “slight variations” depending on your source of information, it is fraught with problems which have been deleted from the model to give it credence. There is little doubt that all opal was not deposited in exactly the same manner. So far I have discovered four different ways in which it has been deposited; there may be more. They all have a common denominator in the equation, ionic-exchange. The fact is that opalization is extremely regional and that large beds of impervious barriers are only secondary to opal formation.

If the only requisite for opalization under the above model were suitable impervious traps, then there would be massive beds of opal over vast areas. However, as every opal miner knows, this is not the case and all opal is unpredictable and patchy in the extreme.

Australian Opal is rarely faceted, nearly always domed. A domed stone is stronger and less prone to chipping. However Opal without a play of color (such as cherry red or amber Opal from Mexico) is often faceted and can look a lot like ruby. Opals are a carver’s delight. The stone is not too hard to work with and will not wear out your diamond tools as much as such stones as Chrysoprase and Agate.

Opal Size

Popular sizes are listed below.

Round: 5 mm and 6 mm

Oval: 6 x 4, 7 x 5, 8 x 6, 9 x 7, 10 x 8, 12 x 10, 14 x 10, 16 x 12, 18 x 13, 20 x 15, 25 x 18, 30 x 22, 40 x 30.

Opal Settings

Jewellery castings for opals can range from very simple designs to very ornate designs with accent diamonds.

There are some interesting myths associated with Opal settings. Some folks believe that Opal Doublets and Triplets are always put into fully backed settings to hide the potch backing. Others believe that full backings are used to protect the stone. Neither myth is true. A jeweler could just as easily “hide” a doublet in an open backed setting, as the join line between the stone and the potch backing would be covered by the side of the setting, making it difficult to see whether the back of the stone was a different stone or simply the less brilliant side of the same stone.

How Opal is Graded and Sold

Rough Opal parcels are sorted into three grades: tops, middle, and low. Each tops parcel has a King stone, which is the best stone in the parcel. Some parcels have several King stones. Color is the primary criteria for grading, but the graders also take into consideration the number of imperfections and faults, and whether a stone is the right shape to be cut into an oval or one of the other popular shapes. You can buy rough opal in several different conditions.

Mine Run

Direct from the mine. The stones have not been cut or ground down. This means that there is more guesswork in the cutting. Purchasing mine run opal can be risky if you are not very experienced.

Off Cuts

Ethiopian Opal Rough

Black Opal Rough

Yowah Opal Rough

Mintabie Opal Rough

The miner has removed whatever black opal rough ,he has a market for and sells you what’s left. With off cuts, you can usually tell what you are going to be able to cut. You must still watch for cracks in the opal because once a crack becomes obvious, a stone can lose half its value.

Rubs

This can often be the best way of buying black opal rough if you are not very experienced. The miner has cut and ground the stones into basic shapes, after having removed most of the rubbish. What you have left is the stone nearly ready for the dopping and polishing process. You have the satisfaction of cutting your own stones without the high risk of buying mine run rough.

Opal is officially sold in troy oz. There are approximately 30 grams to a troy oz and approximately 30-oz to a kilo.

Hints and Tips

Here’s an eclectic assortment of hints and tips you may find helpful when buying or working with opal.

1. Not all Opal is valuable. Potch, is sometimes called “common opal” does not reflect colors. Potch does have its use, however, as a backing for doublets and triplets. When there is color mixed with potch it’s called “potch and color.”

2. Some classes of rough Opal are unsuitable for making gemstones. Some are very porous. Others have a chalky appearance. Still others can lose their color in a short period of time. And yes, it is common for certain types of opals to shatter in a dry atmosphere when the water from the gel evaporates. The best protection for buyers is to purchase opal from a knowledgeable and reputable dealer who accurately details exactly what you are purchasing and is willing to guarantee the stones. Note: If you buy rough opal, there is no guarantee that it will not crack. This is the calculated risk you take and is part of the “excitement” of opal cutting.

3. When examining opal jewellery, look at the stone from the side to determine whether it’s a triplet. Solid and doublet opal domes are simply the opals formed into a curved dome shape. Triplets have clear crystal domes to protect the flat opal surface.

4. It is very difficult to tell whether a stone is a doublet or a solid if it is in a setting. It helps if the back of the setting is open, but it is still no real indication because the back of a doublet and a solid looks very similar, although a doublet is usually perfectly black on the back whereas a solid black opal usually has some imperfections. So if the back of a stone looks too perfect, you have to be a little suspicious. For this reason it’s best to buy opal from reputable dealers who know what they are doing because, unfortunately, many jewelers also are not experienced in opal.

5. When buying opal over the net, ask the seller to give you a summary of how close the graphic sample is to the real thing. Opal is very difficult to photograph and the appearance on the computer screen may give it a different look. Particularly is this the case in regards to size. Graphics come out in all different sizes, usually a lot larger than actual…so make sure you get your ruler out and actually draw the size on a piece of paper so that you will know in your own mind’s eye the actual size of the stone.

6. The same thing applies to the color of the stone. Make sure you get the supplier to give a commentary on the color so that you will know pretty well what you are getting. The camera sometimes picks up colors that are only minimal, while ignoring the feature colors.

Le Opal Body Tone Guide a été conçu et créé par l’Opal Association of Australia et sert de guide pour Black Opal et Boulder Opals . L’industrie des opales a récemment utilisé ce guide pour de nombreux types d’ opales, y compris les opales éthiopiennes .

Body Tone N1 - N4 - Convient aux types de corps de ton noir.

Body Tone N5 - N7 - Convient aux opales de couleur pâle également exploitées à Lightning Ridge, mais ces nuances de corps inférieures peuvent et ont été utilisées sur les opales Coober Pedy et Ethiopian.

Guide de luminosité Opal

Le guide de luminosité d'Opal aide à attribuer une valeur à la luminosité d'une opale à l'œil nu.

Brightest 1 to & Dullest 7 - Peut être utilisé sur n'importe quelle opale avec une luminosité moyenne de 3,5 comme luminosité pour de nombreuses opales de qualité commerciale.

Luminosité inférieure à 3 - Considérée comme une qualité Opale ou de qualité commerciale faible.

Comment est produite la couleur opale?
Formation d'opale

Le nom local donné à potch, tel que grès du désert, dépend de la région dans laquelle ils se trouvent, mais ils appartiennent tous aux mêmes familles de grès feldpathiques, d’argile, de siltstone et de mudstone, qui auraient été déposés sur de grandes zones. ruisseaux et lacs.

Nous appelons tout matériau ayant formé une couleur opale sous le nom de Potch. À l'exception de l'opale volcanique, qui se produit généralement dans les roches tertiaires, tous les types d'opale que nous connaissons si bien se rencontrent dans ces couches. Bien que l'opale se trouve dans les sédiments du Crétacé, il est concevable qu'elle se dépose sur des millions d'années au cours de la période tertiaire. L’enseignement général est que certains minéraux ont été décomposés dans la roche afin de produire de la silice à 120 parties par million, ainsi que des oligo-éléments qui se sont déposés dans des cavités laissées par des os, des coquilles et d’autres matières solubles. Celles-ci, ainsi que des failles et des fractures dans le sol, ont permis aux eaux souterraines d'amener la silice vers la fin de la période de vieillissement.

Bien qu’il s’agisse d’une théorie enracinée, avec de «légères variations» en fonction de votre source d’information, elle se heurte à de nombreux problèmes qui ont été supprimés du modèle pour le rendre crédible. Il y a peu de doute que toutes les opales n'ont pas été déposées exactement de la même manière. Jusqu'à présent, j'ai découvert quatre manières différentes de le déposer. il y en a peut-être plus. Ils ont tous un dénominateur commun dans l'équation, l'échange ionique. Le fait est que l’opalisation est extrêmement régionale et que les vastes couches de barrières imperméables ne sont que secondaires à la formation de l’opale.

Si le modèle ci-dessus était la seule condition requise pour l'opalisation, à savoir des pièges étanches appropriés, il y aurait alors de vastes couches d'opale sur de vastes zones. Cependant, comme le sait tout mineur d’opale, ce n’est pas le cas et toute opale est imprévisible et inégale à la limite.

Black Opal Specimens

Black Opal Pairs

Black Opal Carvings

Black Opal Stones

L'opale australienne est rarement facettée, presque toujours en forme de dôme. Une pierre en forme de dôme est plus forte et moins sujette aux copeaux. Cependant, sans un jeu de couleurs (opale couleur rouge cerise ou ambre du Mexique), Opal a souvent des facettes et peut ressembler beaucoup à du rubis. Les opales font le bonheur des sculpteurs. La pierre n'est pas trop difficile à travailler et n'usera pas vos outils en diamant autant que des pierres comme Chrysoprase et Agate.

Taille Opale

Les tailles populaires sont énumérées ci-dessous.

Rond: 5 mm et 6 mm

Ovale: 6 x 4, 7 x 5, 8 x 6, 9 x 7, 10 x 8, 12 x 10, 14 x 10, 16 x 12, 18 x 13, 20 x 15, 25 x 18, 30 x 22, 40 x 30.

Réglages Opal

Les moulages de bijoux pour opales peuvent aller de dessins très simples à des motifs très ornés de diamants accentués.

Il y a quelques mythes intéressants associés aux réglages Opal. Certaines personnes croient que les doublets et les triplés d'Opal sont toujours placés dans des paramètres entièrement sauvegardés pour masquer le support du potch. D'autres pensent que des supports complets sont utilisés pour protéger la pierre. Aucun des deux mythes n'est vrai. Un bijoutier pourrait tout aussi bien "cacher" un doublet dans un cadre à dos ouvert, car la ligne de jonction entre la pierre et le support de potch serait recouverte par le côté du cadre, ce qui rendrait difficile de savoir si le dos de la pierre était une pierre différente ou tout simplement le côté moins brillant de la même pierre.

Comment Opal est classé et vendu

Les colis bruts d'opale sont triés en trois catégories: haut, moyen et bas. Chaque parcelle de dessus a une pierre de roi, qui est la meilleure pierre de la parcelle. Certaines parcelles ont plusieurs pierres royales. La couleur est le principal critère de classement, mais les niveleuses prennent également en compte le nombre d'imperfections et de défauts, et déterminent si une pierre est la forme idéale pour être découpée en ovale ou l'une des autres formes populaires. Vous pouvez acheter une opale brute dans différentes conditions.

Mine Run

Directement de la mine. Les pierres n'ont pas été coupées ou broyées. Cela signifie qu'il y a plus de devinettes dans la coupe. L'achat d'opale minière peut être risqué si vous n'êtes pas très expérimenté.