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.
The Opal Brightness Guide assists in assigning a value to how bright an Opal is to the naked eye.
Brightest 1 to & Dullest 7 - Can be used on any Opal with 3.5 as average brightness for many opals considered commercial grade.
Brightness lower than a 3 - Is considered a B grade Opal or low Commercial grade.
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.
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.
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.
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.
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.
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.
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.
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.
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Reply Tony translate
Or can we buy the rule Opal base body tone guide SVP
Yes here is link
I do get confused with body tone chart with brightness chart.
Its seems everyone states body tone of an opal but I think Brightness is just as important
Lot black opals now seem to be N2 to N3 as an average
I do not see many N1 black opals and they are all expensive now
That’s why I think Brightness is more important now and most are rated B3 to B4
May I make a suggestion that Brightness be added below Body tone on black opal listings