Evaluation of Opal

Opal is a complex stone to evaluate and this task falls mainly in the hands of experts with years of experience.

There are many 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:

3 Varieties of Natural Opal:

Variety or Type alone does not affect price.

Light Opal	Boulder Opal	Black Opal

Body Tone is the first point of price differentiation; a dark to black background is generally more desirable than a grey or milky-white background. In terms of diaphaneity a transparent/translucent stone will usually be more desirable than an opaque white stone.

Opal Nomenclature Body Tone Scale

 

 

 

 

 

 

 

 

 

Brilliance of colours is of paramount importance – the brightness of an Opal is directly related to price.

Few stones display a brilliant full face of colour throughout 360 degree rotation, some need to be tilted to be appreciated, this is known as directionality. Most stones look best from one particular orientation whereas the finest Opals are non-directional.

Consistency of brilliance, and the density of a stone's body tone, whether it varies over the whole face of the Opal must also be considered.

Estimating the proportion of colour on the stone's face, averaged with the intensity of the colours emitted, one of three categories can be selected: 30%=Subtle, 30-70%=Bright, 70%(+)=Brilliant.

Pattern is almost as important and when combined with brilliance may increase price manyfold. 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 stones have Flash and Broad Flash patterns.

Colours present from the spectral range (Red, Orange, Yellow, Green, Indigo, Violet) may significantly affect an Opal’s value.

Red is the rarest and most valuable colour. Opals displaying red may potentially display all of the spectral colours, and if so they are referred to as ‘multicoloured’.

Bearing in mind that absolute value depends on brilliance, pattern and body tone; red and multicoloured 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.

Sometimes the dominant colours in an Opal are referred to with the post script ‘–fire’ eg. green-fire or multi-fire.

Shape is 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 is maximised in terms of weight and spread, aesthetic talent can then be applied to balance the stone's lapidary design.

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.

Weight is 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.

Some stones have been cut disproportionately, a stone may have been left too thick (heavy on the backside) relative to its spread or face area. Allowance should be made for this when determining the absolute value per carat.

Imperfections or Inclusions are 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 inconsistent patch in the Opal's body or backing that allows light to enter through the back of the stone and so dilute its play-of-colour.

Poor cutting and polishing, in terms of finish and proportions will also significantly reduce a stones value.

 

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 Opal Nomenclature, Australian Gemmologist. 19, 486-496., Anthony G. Smallwood, 1997. Opal Module, GAA course notes, Anthony G. Smallwood, 1998.