Grading

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.

Type 1	Type 1	Type 1	Type 2	Type 3
Black Opal	Dark Opal	Light Opal	Boulder Opal	Matrix Opal
N1,N2,N3,N4	N5,N6	N7,N8,N9	N1 to N9	N1 to N9
opaque	opaque/translucent	opaque to transparent	opaque/translucent	opaque/translucent

Variety? Black, Black Crystal, Semi Black, Dark Grey, White, Crystal, Light Grey, Jelly, Boulder Opal, Boulder Opal Matrix, Yowah Nuts, Andamooka Matrix etc.

Body Tone? The first point of price differentiation; a dark to black background is generally more desirable than a grey or milky-white background.

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 in 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 N1and 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.

Fire? 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.

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.

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 stones 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,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.

Classification of Precious Opal ‘Play of Colour’ is a unique visual phenomenon which sets precious Opal apart from all other gemstones. Also known as 'Fire'; An Opal may display one or more, and sometimes all of the spectral colours. These colours are seen within the grains of a pattern. As the stone is viewed from different directions, rotated and tilted, the colours of each grain may change or disappear.

 

Types & Varieties of Natural Australian Opal

TYPE 1 - NATURAL; 'Light, Dark & Black Opal'

	Black Opal has a blue-black to charcoal body tone and is mainly found at Lightning Ridge in New South Wales. The dark background serves to highlight the colour-play of dramatic spectral flashes. Fine examples of this variety are the most expensive per carat and rival diamonds in price. Black Opal is found as what the miners call ‘Nobbies’, these are fossil replacements of corals or sponges. During its formation, the replacement of organic material by Silica resulted in carbonaceous material or impurities like titanium impregnating the mineral structure giving Black Opal its body colour. (Variety: Black *(pictured), Black Crystal)
	Dark Opal has a smokey to dark grey body tone and may be found at Lightning Ridge NSW. In South Australia Mintabie is famous for Semi Black and Dark Crystal Opal, as are certain fields at Andamooka and Coober Pedy. (Variety: Semi-Black, Dark Crystal *(pictured), Dark Grey, Smokey Crystal, Smokey Jelly)
	Light Opal has a body tone ranging from milky white to transparent. Light Opal is the most common variety found at most fields but mainly at Coober Pedy and in South Australia. White Opal gives the full colour array on an opaque background, whereas Crystal Opal is transparent to translucent without milkiness and has bright colour flashes suspended in its midst. Crystal is superior to white, grey and jelly. Jelly Opal exhibits a moderate play of colour within a transparent background. (Variety: White *(photo), Grey, Crystal, Jelly)
TYPE 1 Body Tone Scale

TYPE 2- NATURAL 'Boulder Opal'

Boulder Opal is found in south west and central Queensland and can be light or dark in appearance. Boulder Opal forms as veins within ironstone concretions and is presented as a solid (one piece) since it is naturally attached to the host rock.
Boulder Opal may display the same darkness and brilliance as gem Black Opal yet it is more affordable and is generally sold at approximately one third of the Black Opal carat price. Boulder Opal is very durable due to the ironstone backing the opal forms on, which the cutter retains and polishes during the lapidary process.

The water content in Boulder Opal is very low, it almost never cracks or crazes as it ages and is considered the most stable type of natural Opal by many experts.

Variety of Boulder Opals: 1) Boulder Pipe 2 ) Black Boulder (green-orange fire) 3) Gem Boulder (split pair) 4) Black Boulder (red fire) 5) Yowah Nut Opal (split-sawn pair)

 

TYPE 3 - NATURAL 'Matrix Opal'

Matrix Opal - is silicified sandstone or ironstone which is intimately diffused with Opal as infillings of pores or holes or between grains of the host rock in which it was formed.

Opal is sometimes encased in or intertwined with the Boulder concretion creating ‘Boulder Matrix’, ‘Yowah Nuts’ and other interesting textures sometimes referred to as ‘picture’ or ‘fun stones’.

1) Light Boulder Yowah Kernel 2) Boulder Opal Matrix

 

Modified & Man-Made Opal

TREATED OR ENHANCED STONES

Treated Matrix - Although some rare examples may actually resemble Black Opal in their natural state, the vast majority of 'Andamooka Matrix' from South Australia, and 'Fairystone' (Sandstone Opal from Queensland) are artificially stained or blackened to enhance their appearance and may also be 'impregnated' with plastic stabilizers to improve the finish.

Heat & Acid Treatment - Since the 1950's miners have treated the rock in sugar solution then carbonizing it by gently heating in concentrated sulphuric acid. Carbon is deposited into its pores by a process involving soaking Opal impregnated with various organic solutions and applying heat up to 500 degrees Celsius.

1)Treated Andamooka Matrix

The rough material is fairly massive in occurrence and has been found across all the Opal fields. In the rough state it is a muddy-grey to orange porous rock and lacks fire, once treated this stone may be startling in quality. This versatile and relatively inexpensive material has applications in jewellery, carvings, clock faces, mosaics, inlays in furniture, tiles, paint finishes etc.

Reputable Gem dealers have a duty to disclose such modifications to the buyer.

MAN-MADE STONES

Composites - Doublets & Triplets are made using a slice of Light Opal and adhering it to a backing of black potch, plastic or boulder.
These are not investment quality stones but generally cheap products aimed as souvenirs. They lack durability and exposure to moisture can result in the opal veneer separating from its’ backing.

Doublets are made by gluing a slice of Opal to a black backing which enhances the reflection of colours from the Opal.

Composites are detectable if the stones’ edges are examined, definite layers can be seen, under magnification.

However some Boulder Doublets may display an irregular or wavy join line as a mixture of powdered ironstone and resin may be used to imitate the irregularity of natural Boulder Opal.

SYNTHETIC STONES

Gilson Black and White Opals have all of the gemmological properties of natural Opal. The discovery of the ordered sphere structure of precious opal led to its synthesis by Pierre Gilson in 1974.

In 1980 Kyocera Corporation of Japan began producing Inamori black and white synthetic Opal. Russian manufacturers are also producing synthetics.

 

IMITATION STONES Made simply by imbedding iridescent foil in opalescent (milky) glass Opal imitations date back to ancient times. These are easily detected on sight. Slocum Stone made of glass (SG 2.45, RI 1.49-1.515) was the best Opal imitation available until 1978 when the Japanese produced plasic simulants that are somewhat more convincing and similar to Gilson to look at. This material has the same refractive index (RI 1.45) as natural Opal however it is lower in density (SG<1.58), often highly porous with a waxy texture.

 

www.bolda.com.au - 100% NATURAL AUSTRALIAN GEMS & JEWELLERY

 

Sources & Image Credits: WITHIN THE STONE, Bill Atkinson, 2004. (Photo.1: Phenomenal - Play of Colour) THE Opaline COLLECTION, Exceptional White, Boulder & Black Opals HANDBOOK OF GEMSTONE IDENTIFICATION, Richard T. Liddicoat, Jr., 1990 NCJV OPAL GRADING & PRICING WORKSHOP, Ralph Pownall, 2011.