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.

La Opal Body Tone Guide fue diseñada y creada por la Opal Association of Australia y actúa como una guía para Black Opal y Boulder Opals . La industria de ópalos también ha estado utilizando recientemente esta guía para muchos tipos de ópalos, incluidos los ópalos de Etiopía .

Tono del cuerpo N1 - N4 - Es para tipos de cuerpo con tonos negros.

Body Tone N5 - N7 : es para los ópalos de color claro extraídos también en Lightning Ridge, pero estos grados de tono de la parte inferior del cuerpo pueden y han sido utilizados en Coober Pedy Opals y Ethiopian Opals.

Guía de brillo opal

La Guía de brillo de Opal ayuda a asignar un valor a qué tan brillante es un Opal a simple vista.

Brightest 1 to & Dullest 7 - Se puede usar en cualquier Opal con 3.5 como brillo promedio para muchos ópalos considerados de calidad comercial.

Brillo inferior a 3 : se considera un grado B, Opal o bajo grado comercial.

¿Cómo se produce el color opal?
Formacion opal

El nombre local dado potch, como arenisca del desierto, depende de la zona en la que se producen, pero todos ellos pertenecen a las mismas familias de arenisca, claystone, limo y mudstone felspathic, que se dice que se han depositado en grandes áreas comprar arroyos y lagos.

Llamamos a cualquier material en el que el color opal se haya formado como Potch. Excepto por el ópalo volcánico, que generalmente ocurre en las rocas del Terciario, todos los tipos de ópalo que hemos llegado a conocer tan bien se encuentran en estos estratos. Aunque el ópalo se encuentra en los sedimentos del Cretácico, es una cuestión de conjetura en cuanto a su depósito durante millones de años durante el período Terciario. La enseñanza general es que ciertos minerales se descomponían en la roca del país para producir sílice a 120 partes por millón, y oligoelementos que se depositaban en cavidades dejadas por huesos, conchas y otros materiales disolubles. Estas, así como las fallas y fracturas en el suelo, proporcionaron caminos para que las aguas subterráneas transportaran la sílice hacia el final del período de intemperie.

Aunque esta es la teoría arraigada, con "ligeras variaciones" según su fuente de información, está llena de problemas que se han eliminado del modelo para darle credibilidad. Hay pocas dudas de que todo el ópalo no se haya depositado exactamente de la misma manera. Hasta ahora he descubierto cuatro formas diferentes en las que ha sido depositado; puede haber más Todos ellos tienen un denominador común en la ecuación, intercambio iónico. El hecho es que la opalización es extremadamente regional y que las grandes camas de barreras impermeables son solo secundarias a la formación de ópalo.

Si el único requisito para la opalización bajo el modelo anterior fueran las trampas impermeables adecuadas, entonces habría camas masivas de ópalo en vastas áreas. Sin embargo, como saben todos los mineros de ópalo, este no es el caso y todo el ópalo es impredecible y en parches en extremo.

Black Opal Specimens

Black Opal Pairs

Black Opal Carvings

Black Opal Stones

El ópalo australiano rara vez es facetado, casi siempre abovedado. Una piedra abovedada es más fuerte y menos propensa a astillarse. Sin embargo, Opal sin un juego de color (como el rojo cereza o el ámbar Opal de México) a menudo es facetado y puede parecerse mucho al rubí. Los ópalos son una delicia para los talladores. No es difícil trabajar con la piedra y no desgastará sus herramientas de diamante tanto como las piedras como la crisoprasa y la ágata.

Tamaño de ópalo

Los tamaños populares se enumeran a continuación.

Redondo: 5 mm y 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.

Ajustes de ópalo

Las piezas de fundición de joyería para ópalos pueden ir desde diseños muy simples hasta diseños muy ornamentados con diamantes de acento.

Hay algunos mitos interesantes asociados con la configuración de Opal. Algunas personas creen que Opal Doublets y Triplets siempre se ponen en configuraciones completamente respaldadas para ocultar el respaldo de Potch. Otros creen que se utilizan soportes completos para proteger la piedra. Ninguno de los dos mitos es verdad. Un joyero podría simplemente "esconder" un doblete en un ambiente con respaldo abierto, ya que la línea de unión entre la piedra y el respaldo de potch estaría cubierta por el costado del escenario, lo que dificultaría ver si la parte posterior de la piedra estaba Una piedra diferente o simplemente el lado menos brillante de la misma piedra.

Cómo se califica y se vende el ópalo

Las parcelas de Opal en bruto se clasifican en tres grados: superior, medio y bajo. Cada paquete de la parte superior tiene una piedra de rey, que es la mejor piedra del paquete. Algunas parcelas tienen varias piedras de rey. El color es el criterio principal para la clasificación, pero los clasificadores también tienen en cuenta el número de imperfecciones y fallas, y si una piedra es la forma correcta para ser cortada en un óvalo o una de las otras formas populares. Usted puede comprar ópalo en bruto en varias condiciones diferentes.

Mine Run

Directamente desde la mina. Las piedras no han sido cortadas ni trituradas. Esto significa que hay más conjeturas en el corte. Comprar un ópalo para correr en la mina puede ser peligroso si no tienes mucha experiencia.