How Does The Back Of An Opal And Its Cut Determine The Value Of An Opal?
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How Does The Back Of An Opal And Its Cut Determine The Value Of An OpalOpal buyers always ask about the strength and durability of an Opal if it has inclusions or sandy potch on the back. We will answer your questions and discuss how the back of the Opal can also affect the value. It is an important question.

It is worth remembering that opal forms in sandstone so you can expect some sand to be in the Opal naturally. Having cut thousands of Australian Black Opals over the last 25 years, I have seen almost every type of backing imaginable. I have operated cutting factories and have noted all the different appearances of the backs of opals. Watching how jewelers set Opal with inclusions and sand on the base of the Opal gives great insight into how it affects the overall stone.

The biggest problem that Jewelers experience is setting cheese block cut (Opals with straight sides). It is difficult to have claws to hold the ring tight or the Opal can be thin. Opal can chip when too much pressure is applied to the ring. The backing of an Opal only causes a problem for Jewellers when there is sand on the sides of Opal. We will go into the things to look for a little bit later.

What is Potch?

Potch on the base of the Opal is the material that has mixed with the silica when formed over millions of years. Potch does not have Opal colours. When water has seeped through cracks in the earth and fills empty cavities sometimes the sides will catch some sand and trap it in the Opal. Other times some dark minerals such as sand and rock can also get trapped. Some potch also has silica in it and is hard.

It should be noted that not all sandy potch is equal. There are different levels of stability. Sand can be fine and loose or it can be hard and stable. There is a special type of sandy matrix called ‘shin cracker’ which is sand mixed with Silica. This type of sand is extremely hard and stable.

Shin crackerblack opal and sand in potch

With my system I value an Opal on different criteria before looking at the back of the Opal. The questions I ask myself when looking at the top of the Opal is to gauge the body tone and brightness as well as the shape and colour intensity.

  • A well cut stone looks appealing and is in balance.
  • A high lustre (quality of the polish) is preferred which highlights the colours.

So in few seconds you have appraised and valued the Opal.

Then I think how quickly will this Opal sell, is it a popular oval shape of freeform shape. Some free form shapes are perfect rhythmical shapes of nature and sell quickly. Others are odd freeform shape and are slow sellers that need good a jeweler to set.

The Next stage of valuing Opal is looking more closely at details.

  • Is the colour directional or full face?
    • If full face of color, the price increases
  • If it has a unique and clean pattern that will increase the value

  • If it is a symmetrical cabochon, the price increases

  • Is it free of cracks and inclusions visible to the naked eye?

  • Lastly I look at the bottom of the Opal.

So if we take a beautiful solid Black Opal from Lighting Ridge as our base Opal and compare it with a Black Opal with a sandy back it may be worth 30% less. If the stone has deep sand in it which makes the stone weaker it could sell for 50% less.

How To Judge The Base Of An Opal

Acceptable Backing

The Opal has a clean black base or meets the following criteria that will slightly have minimal effect in the value of an Opal:

  • Some sand on the back is acceptable
  • Potch on the bottom of Opal has small areas of grey potch on black potch
  • Base has webbing lines

Acceptable backing of an opalAcceptable backing of an opalAcceptable backing of an opalAcceptable backing of an opal

Backings That Will Affect The Value Of The Stone

  • Sandy potch on the side of the Opal. This makes it difficult for a Jeweller to set
  • Opal has a window and part of it is transparent when viewing on top
  • Sandy potch on the side of the Opal will force it to be bezel set instead of claw set
  • Large areas of sandy potch on the base
  • Pitted base with potch. Basically holes in bottom
  • Sandy potch close to where the claws will hold the stone. This might cause the setting to not be strong enough to hold.
  • If it is a crystal stone which is transparent you can see the sand from the face. This usually lowers the value but some people like a 3D effect of the inclusions in the stone.
  • Thin Opal or Opal that has a deep sand in the back will make the stone weak as it may break when knocked.
  • Some jewelers can reinforce the base to add strength like a doublet but this stone must be declared as a treated stone.
  • If a stone is thin the jeweller can set it deep in a bezel setting which protects the stone from knocks.

Example of Backings To Be Aware Of

  • backing of an opal that could cause problemsIf the sand is deep and goes up the side it could cause problems.
  • backing of an opalIf the sand on the back of this Opal is deep it could cause weakness.
  • backing of an opalIt is worth checking that the back of this stone does not contain any cracks.

As you can see the backing of an Opal can affect the value of an Opal but only when there is significant sand or holes. The most important thing to look at first when judging an Opal is the aesthetics from the top. These factors (color, base body tone, symmetry) all contribute to the final price of an Opal. The backing plays a minor role in valuing the final Opal.

Now That You Know What To Look For Check Out Our Opals


值得记住的是, 蛋白石是在砂岩中形成的,因此您可以期望某些蛋白石自然地存在于蛋白石中。在过去的25年中,我砍掉了成千上万的澳大利亚黑蛋白石 ,几乎可以想象到每种支持。我经营着切割工厂,并注意到蛋白石背面的所有不同外观。观看珠宝商如何在蛋白石的基础上用夹杂物和沙子来放置蛋白石,可以深入了解其如何影响整体宝石。



蛋白石基上的Potch是在数百万年的形成过程中与二氧化硅混合的材料。 Potch没有蛋白石颜色。当水渗入土壤中的裂缝并填满空洞时,有时它们的侧面会夹住一些沙子并将其捕获在蛋白石中。其他时候,一些深色的矿物,例如沙子和岩石,也可能被困住。一些罐子里也有二氧化硅并且坚硬。

应该注意的是,并不是所有的沙坑都是平等的。有不同级别的稳定性。沙子可以是细小且松动的,也可以是硬而稳定的。有一种特殊类型的沙质基质称为“ shin cracker”,它是与二氧化硅混合的沙子。这种沙子非常坚硬和稳定。



  • 切得好的石头看起来很吸引人,而且平衡。
  • 优选高光泽(抛光质量)以突出颜色。




  • 颜色是定向的还是全脸的?
    • 如果全脸变色,价格上涨
  • 如果它具有独特且干净的图案,将增加价值

  • 如果是对称凸圆形,价格会上涨

  • 它是否没有肉眼可见的裂缝和夹杂物?

  • 最后,我看蛋白石的底部。

因此,如果我们以Lighting Ridge的美丽坚固的黑欧泊作为我们的基础欧泊,并将其与带沙背的黑欧泊进行比较,则价格可能会降低30%。如果石材中含有深沙,这会使石材变得更弱,则可以将其售价降低50%。




  • 背面可以放些沙子
  • 蛋白石底部的Potch在黑色Potch上有一小部分灰色Potch
  • 底座有织带线



  • 蛋白石一侧的桑迪·波奇。这使珠宝商难以设置
  • 蛋白石有一个窗口,在顶部查看时它的一部分是透明的
  • 蛋白石侧面的沙坑将迫使其设置为挡板设置,而不是爪设置
  • 底座上大面积的沙坑
  • 锅底开槽。基本上在底部有孔
  • 沙坑靠近爪子将石头固定的位置。这可能会导致设置的强度不足以容纳它。
  • 如果它是透明的水晶石,则可以从脸上看到沙子。这通常会降低价值,但有些人喜欢石头中内含物的3D效果。
  • 薄蛋白石或背面深沙的蛋白石会使石头变弱,因为敲打时可能会破裂。
  • 一些珠宝商可以加固底座,以增加强度,如双层装饰,但必须将此石材声明为经过处理的石材。
  • 如果宝石很薄,珠宝商可以将其设置在挡板设置的深处,以防止宝石受到撞击。


  • 可能会引起问题的蛋白石的支持如果沙子很深并向上爬,可能会引起问题。
  • 蛋白石的支持如果此蛋白石背面的沙子很深,可能会导致虚弱。
  • 蛋白石的支持值得检查的是,该石头的背面没有任何裂缝。



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