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You are here: Physical > Grading Gemstones > Opal Grading

Grading Opals

Many factors play a part in determining the value of an opal;  the type of opal, body tone, brilliance, pattern, colour bar thickness, the play of colour, the quality of the cut & polish, and the size of the stone. Faults in the stone also impact the price. 

Types of Opal

Black Opal
Black Opal - Black opal is characterised by a dark body tone causing brightness of colour which is unmatched by lighter opals. Black Opals are usually mined in Lightning Ridge, New South Wales, and are the most famous, and sought-after type of opal. The term 'black opal' does not mean that the stone is completely black (a common mistake), it simply means the stone has a dark body tone in comparison to a white opal. 

White Opal
White Opal - Also known as 'milky opal', white opal features light white body tones, and is mined in South Australia. White opal is more common and because of its body tone, generally does not show the colour as well as black opal. Nevertheless, white opals can still be absolutely magnificent in colour if a good quality stone is found.  

Boulder Opal
Boulder Opal - Boulder opal forms on ironstone boulders in Queensland. This type of opal is often cut with the ironstone left on the back, as the opal seam is usually quite thin. Leaving the ironstone on the back means that boulder opal can be very dark and beautiful in colour. The opal forms within the cavities of the boulders in both vertical and horizontal cracks. Boulders vary in shape and size, from as small as a pea, to as big as a family car. Boulder Opal has a tendency to cleave; when cleaved the "split" leaves two faces of opal, with a naturally polished face.  

Crystal Opal
Crystal Opal - Crystal opal is any of the above kind of opal which has a transparent or semi-transparent body tone - i.e. you can see through the stone. Crystal opal can have a dark or light body tone, leading to the terms "black crystal opal" and "white crystal opal".

Matrix Opal

Matrix opal - Matrix opal is where the opal occurs as a network of veins or infilling of voids or between grains of the host rock (ferruginous sandstone or ironstone). Matrix comprises precious opaline silica as an infilling of pore spaces in silty claystone or ironstone. It generally shows fine pinfire colour in the natural state.

Andamooka matrix opal may be enhanced by soaking the specimen in a sugar solution and then boiling in acid to deposit carbon in the available pore spaces, resulting in a dark background.

Natural boulder opal matrix is another kind of matrix opal, found at Yowah in Queensland, which in its natural state consists of brown ironstone with small deposits of opal interspersed. This kind of opal is not treated.


Yowah Nut
Yowah nuts - Found in the far South Western mines at Yowah in Queensland, Yowah nuts are ironstone concretions resembling 'nuts' which contain precious opal in their centre. Upon cracking or slicing the Yowah nut, the precious opal is revealed.

Opal Class

The first step taken in grading opal is to identify the type of opal which is being valued. An opal doublet or triplet can be worth considerably less than a solid opal. Doublets and triplets are an 'assembled' stone which only contains a very thin slice of natural opal and are therefore generally much less valuable.

Although opal cutters prefer to cut and polish opals as a one, solid stone, in many instances that is just not possible to achieve. Sometimes the opals are too thin and fragile to stand on their own and to be mounted into a piece of jewellery. To overcome this problem opal jewellers use a relatively simple and very effective technique and create doublets and triplets.

 Jewellers choose a black backing (usually colourless, dark, natural - but not precious opal called potch, black glass or a piece of hard, black plastic) and simply attach a slice of opal on top of it with black cement or some other  kind of adhesive.

The final product is called doublet. It looks very appealing because the black backing part causes the body of the opal to look much darker and automatically all other colours become more vibrant and visible.

Doublets can be easily recognised by looking them from the side.    A fine, very straight line, dividing the opal part from its backing, should be visible. However, once set into a jewellery piece it is almost impossible to tell the difference between a doublet and a solid opal.

Triplets are made in exactly the same way as doublets but usually from much thinner opal slices. As thinner slices are much more fragile they need some sort of extra protection.

In those cases a clear piece of glass, quartz or plastic in a dome shape is glued on top of the opal and a triplet is born.

Doublets and triplets are much cheaper alternative to solid opal gemstones. Doublets generally contain more real opal than triplets. Triplets with a very  similar appearance to solid opals might be 10-15 times cheaper in price.

Both doublets and especially triplets should never be exposed to or be kept under water for very long period of time. Water might weaken the adhesive components and somehow can find its way in between the layers of doublets and triplets and can cause some serious damage.

Colour Play

The phenomenon known as the "play-of-colour" is the brilliant range of the full spectrum of colours caused by the diffraction of white light by the internal structure of orderly arrayed spheres of silica. Red (fire) opal is generally more valuable than a mainly green opal which, in turn, is more valuable than a stone showing only blue colour. Nature does not produce a red colour as often as it does a blue or green. Red colouring is caused by larger microscopic silica spheres, whereas blue is caused by the more common small spheres.

The dominant colour has value in this order: red, orange, yellow, green, blue, indigo and violet. The red orange colours are the more valued. This rule is more particular to black opal, 'red on black' being the most valuable.

Brilliance

Brilliance refers to the brightness and clarity of the colours displayed by opal, when the stone is viewed face-up. This ranges from brilliant , bright , to subdued or dull.

Pattern

The pattern of coloured segments, forming the play-of-colour of a precious opal, is unique to every individual opal. The distinctiveness and colour displayed by these segments determines the quality of the pattern of an opal.

Excellent patterns include;

  • Harlequin - large sections of colour in which each colour segment is roughly the same size and shape, like a mosaic or chequerboard. A true harlequin pattern is extremely rare and highly sought after.
  • Flagstone - large sections of colour with straight edges, in a random pattern
  • Ribbon - narrow, parallel cascading lines of rolling colour
  • Straw - random thin strips of overlapping colour
  • Chinese Writing - thin strips of overlapping colour which resemble Chinese characters
  • Picture stones - the intriguingly unique patterns of 'novelty' or 'picture' stones, which resemble an object, landscape, animal, person, etc.

 

Good patterns include;

  • Floral - a random pattern of colour with good spread
  • Rolling Flash - large sections of colour which roll across the stone as it turns
  • Broad Flash - large sections of colour which flash as the stone turns
  • Pinfire - tiny points or specks of colour

Colour Bar

The thickness of the colour bar in opal is relative to the overall size and shape of the individual stone. Boulder opal typically has a very thin colour bar due to the way the opal is geologically formed. This should be taken into account when valuing the stone, however makes little difference to its appearance once set in jewellery.

Black opals and black boulder opals with a very thin colour bar are frequently worth many thousands of dollars a carat, although they would be worth more if they had exactly the same appearance with a thicker colour bar. Thickness of bar would also enable a cabochon to be cut from the rough, thereby increasing value.

Shape

Oval, pear and other regular shaped stones will bring a higher price than irregular shapes (free shapes or free form). Although it is possible to obtain greater yields from free shapes, the demand is higher for the regular shapes. A cabochon enhances the appearance and signifies a thicker colour bar. A domed stone is therefore more valuable than a flat stone. Black opals do not usually have a cabochon.

Faults

Faults which can detract from the value of a finished opal are many and varied. A crack in the face can render almost worthless an opal that otherwise might have been worth a considerable amount per cart. Crazing, i.e. many small cracks in the opal's face will also relegate the stone to worthless. Sand and various other minerals can be found as inclusions in and/or under the colour bar, and in the potch of opals. Other faults include potch lines, webbing, (grey lines) and windows (sections devoid or lacking in colour). The consistency of colours and pattern when viewed from different directions also has an influence - when a stone "won't face", the colour only shows through on certain angles and otherwise has little colour. The visibility of potch or brown ironstone on the surface of the stone will also lead to a drop in value.