Grading

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.

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.

broadflash	chaff/straw	ribbon	rollingflash
floral	straw (all blue)	harlequin	pinfire
flagstone	ribbon & floral	hexagonal harlquin	chinese writing

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.

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

Classification of Precious Opal ‘Play of Colour’ is a unique visual phenomenon which sets precious Opal apart from all other gemstones. 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.

Each variety is a classification determined by background colour or 'body tone' which unlike the spectral colours do not appear to originate from coloured lights within the Opal. Body tone is produced by the same mechanism that produces the colours in pigments and paint. Each variety of Opal may include several types as outlined:

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. (Types: white *(photo), grey, crystal, jelly)

Black Opal has a blue-black to dark grey 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. (Types: solid black *(photo), black crystal, semi-black)

Boulder Opal is found in south west and central Queensland and can be light or dark in appearance, it forms naturally within the veins of ironstone boulders.
Boulder Opal may display the same darkness and brilliance as gem Black Opal yet it is more affordable and is generally sold at one third of the Black Opal carat price.

Many unique examples of Boulder opal are encased in or intertwined with the Boulder mother stone creating ‘matrix’, ‘Yowah nuts’ and other interesting textures known as ‘picture’ or ‘fun stones’.

	Types of Boulder Opals: 1) Boulder pipe 2)Black Boulder green-orange fire 3) Light Boulder Yowah Kernel
	4) Black Boulder red fire 5) Gem Yowah Nut Opal split-sawn pair 6)Boulder opal matrix 7) Gem Boulder split pair

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 variety by many experts.

Treated Opals

Treated Opals include Andamooka Matrix and Sandstone Opal from Queensland, referred to by the miners as Fairystone, which is often stained or blackened to resemble black opal and may also be treated with plastic stabilizers to improve the finish. Since the 1950's miners have treated the rock in sugar solution then carbonizing it by gently heating in concentrated sulphuric acid. 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. Carbon is deposited into its pores by a process involving soaking Opal impregnated various organic solutions and applying heat up to 500 degrees Celsius. The rough material is fairly massive in occurrence and has been found across all the Opal fields. It is relatively inexpensive and has versatile applications in jewellery, carvings, clock faces, mosaics, inlays in furniture, tiles, paint finishes etc.

Matrix or Sandstone Opal never transmits light and may be detected by its porosity, lower hardness and lower density (SG). Under magnification tiny black spots can be seen in the surface colour of these stones and poorly treated examples may show uneven patches of body tone.

Man-made Opal 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.

Triplets are made by adding a magnifying crystal top to a thin opal doublet.

Composites are detectable if the stones’ edges are examined, definite layers can be seen, under magnification. But some Boulder Doublets may display an irregular join line. If a pin is heated with a naked flame and then pushed into the join line, it should penetrate easily as the backing which looks like ironstone is actually powdered ironstone in an epoxy resin.

Synthetic or 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, however there are several points of detection for these man made products;
• Freedom of inclusions like potch or sand, and a stone which looks too perfect.
• Gilson White Opal does not fluoresce under UV light unlike most natural white Opal.
• Very regular play of colour, with colours always arranged in uniform layers on vertical columns may arouse suspicion. Under magnification this has the effect of resembling a ‘snakeskin’ or 'mosaic' pattern.

Imitation Opals made simply by imbedding iridescent foil in opalescent (milky) glass 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.

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