Black opal is produced in a variety of grades from the relatively inexpensive up to around $20,000 per carat for the rarest, most exquisite gems.
The quality and value of black opals is governed by a number of factors:
Brilliance or lustre is the intensity of colour play within the stone. This refers to the amount of light that an opal reflects and the hue it reflects. It is one of the main factors determining the quality of black opals. More valuable opals show brilliant flashes of colour while lesser-quality opals have dull or subdued colours.
In a finest quality black opal this colour play should be clear and crisp, the shades of colour should be distinct and definite, and no cloudiness or film should be present. To assist in judging brilliance, several stones should be compared.
Colour Background (Body Tone)
The background or body colour of black opal, over or within which the spectral colours occur, may be mid to dark grey, very dark blue, or can range through to black. Most dark grey opal is correctly referred to as black opal, while those with a mid-grey body colour are known as "semi-black". An intense black background is the most desirable. However dark grey forms the majority of opal mined, and is readily acceptable.
Black opals are very rare and therefore the most valuable. White opals, the most common opals, often hold less value. Light grey, grey, and semi-black opals fall in the middle in terms of value.
Spectral Colour Range (Play of Colour)
The colour play within a black opal, and the way in which the spectral colours move and interchange, is also important. The colour of an opal is variable, ranging across the visible spectrum from violet to deep red. Opals that flash red, in particular, violet/purple, hold a higher value than more common opals that have a green or blue hue. Pure, fully saturated colours are preferable to pale, less intense ones.
Pure blues are desirable and attractive, and the least costly of the black opals because of their relative abundance. The shades range from milky blue to deep sapphire blue and a stunning electric blue.
Blue–black opals are superb as pearl clasps, with the deep royal blue contrasting vividly with the silvery lustre of the pearls.
A small amount of green occurring in an otherwise blue opal greatly increases its value.
Clear intense greens over a dark background are very beautiful. Most of the black opals mined are green, green-blue, or blue and hence they form a very important section of the market.
A conspicuous orange component occurring in an otherwise green stone raises its value considerably.
This is rarely found in black opals as a clear lemon yellow. It is usually a golden colour combined with a pale orange or green.
This is a very desirable colour, second only to the red tones, and occurs in only a small percentage of all black opals. When the orange is rich, deep and saturated, and especially when it is predominant and associated with small amounts of red and green, it becomes a high quality and valuable gem.
The various tones of red, from magenta through scarlet to crimson, are the most sought-after colours and include a lovely burgundy red, which has great appeal. A saturated scarlet to crimson colour play over a completely black base, devoid of greyish component, yields the famous "red on black" which is visually quite startling, and is the finest and rarest black opal.
When "red on black" combines in a pleasing way with a minor array of other colours, especially green, orange and blue, it presents the finest and rarest of gems.
Black opals are famous for the patterns which occur, sometimes in spectacular forms. Many different colour patterns are observed as the gemstone is turned or viewed from different angles. Terms such as harlequin, pinfire and flash, describe the unique interplay of colour in an opal.
- Harlequin — A harlequin pattern is made up of a patchwork of irregular-sized squares of colour.
- Pinfire — A pinfire pattern consists of closely spaced pinpoints or specks of colour.
- Flash — A flash pattern shows broad irregular flashes of colour which change or disappear as the stone is rotated.
Many black opals combine various patterns. People often select names of their own to describe unusual ones.
This term is used to indicate whether or not an opal, when viewed face on, presents its colour directly to the viewer. The stone's value is less if its splendour is visible only from acute angles.
Many fine black opals, especially those with broad flashes of colour, are at their best when viewed from a slight angle. These have special appeal when used as pendants or ring stones.
Some black opals are described as having character, which means their appeal is far greater than others of the same technical standard. Every person sees opal in their own way, and an inexpensive stone can often have great personal appeal.
The fact that every black opal presents some different characteristic is very important. Unlike other gemstones such as sapphires, rubies or emeralds, each black opal shows unique colour combinations. It is rare to see two similar stones, and impossible to find two which are identical.
Opals with more attractive and appealing colour combinations have higher values. The colour patterns of many black opals appear as though they could have been painted by a master.
The most sought-after shape for an opal is a cabochon or domed oval, the length of which is about 40% greater than the width, and the height (dome) equal to about 60% of the width. In practice this is very difficult to find, because opals have been fashioned irregularly by nature.
Perfectly shaped ovals generally command a premium price, because valuable opal material is sacrificed when cutting them to this form. In most cases about 40–60% of the rough stone is ground away to produce the finished article.
Many people prefer the appearance of rounds, elongated ovals or irregular shapes. Some opals are deliberately cut this way when the material is of exceptional value, or of unusual interest, in order to preserve precious opal. Some designers select these shapes to create distinctive jewellery of great individuality.
Stones which are flat or have a very low dome are less expensive than a well-domed opal of otherwise similar quality. Extreme thickness or thinness could reduce an opal's value.
The colour layer of many black opals is quite thin, the bulk of the opal being potch (opal without spectral colour). Very few black opals have colour extending through the full body. This thin colour layer is quite normal. When the precious opal formed in its host rock in the geological past, the conditions necessary for the growth of precious opal with colour, instead of potch, were extremely rare.
Black opals of between 0.75 and 10 carats weight are the most popular because they are very suitable for rings. Stones of this weight and heavier are also used in pendants and brooches. Very large black opals, of 20–30 carats or more, can be made into spectacular items of jewellery.
The value of black opals does not normally increase more than proportionally with size, as is usually the case with stones such as diamonds. Opal has a lower specific gravity than most other gems and is therefore larger than a sapphire, ruby or emerald of the same weight.
Imperfections (or faults) that impact on a finished opal are many and varied and will greatly influence the valuation of the opal. However, fault penalties are perhaps the most difficult factor to decide.
Imperfections include cracks or crazing; inclusions; sand, gypsum or potch in the face of the stone; colour not facing; or poor cut, shape or polish.
The gemstones should be free from obvious flaws in the face of the opal.
Gypsum sometimes occurs as a web of fine grey lines on the surface, which if clearly apparent will reduce the value.
Many black opals have flecks or pockets of sand in the body, or on the back of the stone where they are not visible from the face. This is quite normal and does not affect the value of the opal. If however the sand occurs on the face, it is a definite flaw.
Many opals, especially those with a grey or dark background, have paler areas showing through the colour. These are usually the result of the potch being unevenly coloured at the back of the stone, or of sand intruding from the back into the colour layer. These will reduce the opal's appeal, and hence its value.
Natural opals examined by an expert should come with a certificate of authenticity outlining these various qualities.
The Lightning Ridge Miners Association (www.wj.com.au) Opal Advisory Service also provides advice on price, as well as cutting and market trends. As a general rule, each miner is able to have up to 6 cut and polished stones valued at a time. The LRMA Opal Advisory Service can also provide digital photographs of your opal. Having images of your opals on your CD or floppy disk can have numerous advantages in today's market place.
The LRMA Opal Advisory Service operates every Saturday between 9:00am and 11:00am.