Opal is a complex stone to evaluate and this task falls mainly in the hands of experts with years of experience. There are eleven more facets to an Opal than the 4C's (Colour, Clarity, Cut, Carat) used to grade a Diamond. The factors outlined below provide a concise guide to the foremost influences on an Opals value:
Type of Natural Opal? Type or Variety alone does not affect price.
opaque to transparent
N1 to N9
N1 to N9
Variety? Black, Black Crystal, Semi Black, Dark Grey, White, Crystal, Light Grey, Jelly, Boulder Opal, Boulder Opal Matrix, Yowah Nuts, Andamooka Matrix etc.
Opal Nomenclature Body Tone Scale
Transparency? Light Opal of the 'Crystal' variety is found on most Opal fields and is transparent. A dot placed on the back of a stone using a black felt pen should be clearly visible from the face. Black Crystal may be transparent to translucent when held up to the light however it is exempted from the above test due to body tone. The majority of solid Opals are translucent, transmitting and diffusing light so that objects beyond cannot be clearly seen. Crystal Opal from South Australia, particularly in red stones, may posses superior hues of rich red. However the clarity and purity of Andamooka Crystal, coupled with brilliance and relative rarity, make it the more valuable stone quality for quality. Most Lightning Ridge Crystal is cut into a pleasing high dome which enhances the depth of pattern. A transparent Jelly Opal or Crystal Opal will usually be more desirable than an opaque White 'Milky' Opal. Conversely Black Opals with an N1 or N2 body tone are opaque and may fetch the highest prices acheived per carat.
Brightness? Vividness of colours is of paramount importance – the brightness of an Opal is directly related to price. It is not uncommon to observe several levels of brightness in the face and variation in a stone's body tone. An assesment is made on the overall impression of a stone. One of three categories can be selected: 30%=Subtle, 30-70%=Bright, 70%(+)=Brilliant.
Directionality? Relates to colour display and how the hues are arranged on the face of a stone. When rotated through 360° in 90° segments most opals will show a marked difference at each turn. Many showing a good play of colour at one angle, can be nearly blank from one or two other angles and a price penalty is imposed dependent on the severity of the characteristic. 'Broad' or 'Flash' pattern stones often display this characteristic. Most stones look best in one particular orientation, some need to be tilted to be appreciated whereas the finest Opals are non-directional.
Distribution? Opal quality is markedly affected by the ‘depth’ of pattern and amount of 'fire' showing on the face. This factor is judged from the stone’s best angle. Very few, with the exception of 'Flash' pattern stones, will show 100% of the face covered in colour. Consequently a sparse ‘distribution’ of colour attracts a lesser value.
Saturation? Refers to the dominance of hue in the fire colour. Rich saturated hues; emerald green, rich orange etc are always present in the finest gem quality stones yet even pastel tones if bright can make a top quality stone. Conversely subdued brightness and pale pastel hues can only indicate, at best, a commercial quality stone. The finest Opals hold their beauty in all types of light.
Fire? (Aka. Play-of-colour) The dominant hues in an Opal are referred to with the post script ‘-fire’ eg. green-fire or multi-fire (multi-coloured). The spectral colours (red, orange, yellow, green, blue, indigo, violet - in order of value) are usually combined in varying amounts in most Opals. The predominant hue is listed first with the main secondary hue following, eg. green-blue. The predominant hue should be at least 50% to 70% of the total fire (play-of-colour), the second or supporting hue should be around 20% or more.
Some combinations are far more attractive than others, eg. red-blue or green-blue when hues are intense, while red-orange and green-multi-fire are not as desirable. Gem quality stones with a predominance of red or orange are enhanced by a complimentary touch of vivid blue.
Red Opal is the rarest and most valuable and a predominantly red stone may potentially display all of the spectral colours, if so it may be referred to as ‘multi-red’. Bearing in mind that absolute value depends on brightness, pattern and body tone; red and multi-coloured Opals are rarer than green-orange, blue-green and blue Opals in that order. Given all other factors are equal an Opal containing red can be valued greater than a blue-green-yellow Opal by a factor of up to 3 times.
Pattern? Almost as important and when combined with brilliance may increase price manyfold.
straw (all blue)
|flagstone||ribbon & floral||hexagonal harlquin||chinese writing|
Generally a larger pattern is more valuable than a smaller pattern eg. 'Harlequin’ is the most highly prized pattern whereas ‘Pinfire’ is a more common pattern. The descriptions may be termed otherwise, eg. 'Broadflash' may be called 'Peacock' pattern, and the 'Cat's Eye' phenomenon is usually referred to as 'Rolling Flash' by the Opal trade.
A vivid pattern is more valuable than a static less playful one. A lively stone posses depth or saturation of colour and pattern. While every Opal has a unique pattern, there are seven categories of patterns that all Opals fit within: Pinfire, Flash, Broad Flash, Rolling Flash, Harlequin, Rare Patterns and Picture Stones. Over 90% of cut stones have either Flash or Broad Flash patterns.
Shape? Most often dictated by the rough form. Most Opals, particularly Black Opals, tend to be fashioned as ovals and because Opal is cut ‘en cabochon’ or with a domed surface these features have traditionally been preferred for jewellery aesthetics and calibration purposes.
However, most Boulder Opals are cut as free shapes which can lend themselves to more distinctive designs. In the last decade there has been a strong trend towards cutting freeform 3-dimensional shapes from most gem quality Opal. By sculpting the rough, yield may be maximised in terms of weight and spread, aesthetic talent must be applied to balance the stone's lapidary design.
Proportion? A stone with a domed surface will be more valuable than one with a flat or undulated surface. This is because the domed stone has more depth from which to emit play-of-colour. The ratio of the colour bar with play-of-colour to the thickness of potch or ironstone backing should be balanced and have a suitable setting edge for jewellery manufacture. Poor cutting and polishing will significantly reduce a stone's value. Some stones have been cut disproportionately, a stone may have been left too thick (heavy on the potch or ironstone backside) relative to its spread or face area. Consideration is made of a stone's proportion and aesthetic balance when determining the absolute value per carat.
Inclusions? Not uncommon in the back of stones, generally these are small sand-spots and do not affect price drastically. However marks or cracks that are noticeable in the face of the stone will have a marked effect on the price of an Opal. Visible inclusions may include; patches or lines of potch, 'webbing', 'sand spots', crystals of gypsum and ironstone in the face of Boulder Opal. 'Windows' in Black or Boulder Opal where there is an area of transparency in an opaque Opal's body that allows light to enter through the back of the stone and so dilute its play-of-colour are detrimental to price. Certain types of sand and other inclusions are indicators of origin.
Origin? Australian Opal mining fields are at:
Adavale, Andamooka, Beechworth, Coober Pedy, Coocoran, Grawin, Jundah, Koroit, Kynuna, Lambina, Lightning Ridge, Mintabie, Opalton, Queensland, Quilpie, Sheep Yard, South Australia, Tintenbar, White Cliffs, Winton, Yaraka and Yowah.
Weight? Measured in Carats (1carat=0.2 Grams; 1kg=5000carats).
Prices per carat are generally at their greatest for exceptional stones between 3 and 5 carats and up to about 10 carats, after which larger sizes may become less commercially viable for jewellery purposes and value per carat tends to decrease.
Sources & Image Credits:
A JOURNEY WITH COLOUR, Vol I & II, Len Cram (Photos: various patterns)
AUSTRALIAN PRECIOUS OPAL, Archie Kalokerinos, 1971.(Photos:Hexagonal Harlequin, Pinfire)
AUSTRALIAN PRECIOUS OPAL, Andrew Cody, 1991. (Photos: Brilliance Scale)
OPAL IDENTIFICATION AND VALUE, Paul Downing PhD., 2001.
Crystal, Boulder and Black Gems from the Opaline Collection
A NEW ERA FOR NOMENCLATURE, Australian Gemmologist. 19, 486-496., Anthony G. Smallwood, 1997.
OPAL MODULE, GAA COURSE NOTES, Anthony G. Smallwood, 1998.
THE SMART CHART, Peter Evans, 2005.
NCJV OPAL GRADING & PRICING WORKSHOP, Ralph Pownall, 2011.