An Opal is a gemstone over 90 percent of which are found in South Australia although small deposits are also found in some other countries such as Mexico, The Nevada desert in the USA, Brazil and a few others.
It is a non crystal substance and formed from silica. It is related to quartz and agate and formed from ‘lumps’ of silica rather than the usual faceted crystals.
It can usually be found at the site of old thermal springs where the minerals have risen and lined caves, cracks in the earth and vents and then cooled down. Most of the opals you will find therefore are many millions of years old. Some say as old as the dinosaurs, although much more friendly of course.
The quality about opals that makes them attractive and much sort after is their refractive and reflective qualities. The term opalescence was coined to describe this phenomenon where light is refracted within the stone as a result of imperfections and arrangements of silica within the stone. The size and arrangements of these bits of silica are what cause the interplay of light, similar to a rainbow effect caused by a prism breaking up the light but much more dramatically.
Opals are a valuable gem and worth millions of dollars in exports for Australia. Everywhere you go in the cities and tourist areas you will find heaps of Opal shops.
Choosing the right Opal
There is no set price of opals. The price depends on such factors as, rarity, quality of the opal, international market forces such as supply and demand etc.
The supply of opals depends basically on how many are dug up out of the ground. This varies year to year of course but the quantity does not vary very much. There is a fairly steady supply of opals year to year
Demand tends to relate, however to the enthusiasm of the customer for opal, and may therefore be influenced by fashion and market trends.
An example being the demand for fine quality opals by the Japanese has caused the prices of such opals to increase very well.
Choosing the right opal really depends, thought, on how you intend to wear the opal, Will it be a ring? Or a Broach, or in a necklace, for example. Some opals look better head on while others may show more color when viewed at an angle.
Valuing opals depends on a number of factors. The attractiveness and appeal of a particular opal. The type, color, size as well as soundness of precious opal are also important factors in determining the price paid for the gemstone.
The price of an opal is also based on the quality of an opal. This is usually expressed as a per carat in weight. More on weight later but there is a real difference between an uncut unpolished opal and a cut and polished opal.
Some of the determining factors include:
Body Tone/Color. A Black or dark opal is usually considered more valuable than a white or light opal. Opals are rated according to their Body Tone “N” Value from 9 (black) down to 1 (light transparent)
Play of Color. The coverage of the play of colors is important and there should be no patches of inferior colorless opal, called potch, in a gem class or high grade opal stone.
Colors in order of value for an Australian Opal are:
Opals are sold by carat weight. One carat is equal to 200 milligrams in weight.
However, Rough Opal, in Australia is normally sold by the ounce:
1 Troy ounce = 31.103 grams or 20 pennyweights.
1 Pennyweight = 1.555 grams.
155.5 Carats to the Troy ounce.
The weight used in the USA is:
1 Avoirdupois ounce = 28.35 grams
1 Avoirdupois ounce = 0.9116 Troy ounces
Opal Dominant Fire Color
As far as dominant fire color is concerned the clarity of the color is the critical issue when assessing the value of a opal. The most rare color is called Red Fire followed by the green/ orange, green blue and then blue. Red Fire is considered the most valuable followed by the rest in sequence.
Brilliance and the pattern of the colors also play a part. Here the intensity makes a difference and can influence the price.
But the brilliance and clarity of an open proportioned pattern play the biggest part in determining the price here. A brilliant blue/green can be worth more than a dull Red Fire or a lackluster black opal.
So the degree of brightness is highly important in so far as an opal will have little value without it.
This is so important that a glowing average opal can be worth more than a dull stone with an excellent pattern, such as a harlequin.
Types of Opal
One should have some knowledge of the types of opal. Most are listed on this site but basically you have black boulder opals, black opals, white opals, solid opals and others.
It is noteworthy that solid opals are those which have been mined and presented in their natural state, after being cut and polished. They are not chemically treated as some opals are and cracks are not filled with cement.
An opal is made up of close packed aggregates of silica spheres with a water content between 3-10 percent and in precious opal the arrangement of spheres is in orderly layers. Light passing through the spheres is diffracted at the void and layer interface to produce a vivid play of color associated commonly with opal.
The larger silica spheres are normally associated with more sought after colors, such as red.
Color is everything in Opals. The more color, the more expensive the opal. Reds, pink, yellow and orange are harder to find than the green, blue or violet and therefore and therefore more expensive.
A solid precious opal is more valuable than doublet or triplet opals.
A Black opal is more valuable than boulder opal is more valuable than light opal,
A good premise is, the darker the body color, the more valuable the gem.
A stone with strong color and a full spectrum range is generally more valuable than one with a predominant red fire, which is more valuable than one with predominantly green color which in turn is more valuable than a stone showing only blue color.
Opal Diffracted Color
A lot of care should be taken when applying this factor to assessing an opal. Remember that brilliance overrides all other factors. A blue/green brilliant stone will usually be more valuable than a dull red stone. The dominant color has value in the order of red, orange, yellow, green, blue, indigo and violet. The red orange colors are the more valued. This rule is more particular to black opal, “red on black” being the most valuable.
Opal Base Color
This is determined by looking through the top of the Opal beyond the play of color. Using a jewelers loupe is a good idea here.
Opal Body Color
When it comes to black opals or black boulder opals, the degree of darkness in the body color should be taken into consideration. The more black the body color the more valuable the gem will be.
Opal Play of Color
The play-of-color of the opal can be the most difficult factor to judge for any opal and it does make a difference to the price for the end user.
Brightness. How bright is the overall play-of-color?
Spectral range. What is the range of colors visible in the play-of-color ?
Saturation. How pure and vivid are the colors forming the play-of-color?
Pattern. What is the size, shape, regularity and rarity of the play-of-color?
Consistency. Is the play-of-color, pattern, brightness consistent or variable over the whole face of the opal?
Directionality. Is the play-of color visible from all directions as the opal is rotated?
This term is used to indicate if an opal, when viewed face on, presents its color directly to the viewer. The value of the stone will be less if its’ splendor is only visible from an acute angle.
Yet there are many opals, particularly those with broad flashes of color, that are at their best when viewed from a slight angle. These can have special appeal as pendants or ring stones.
Opal Brightness of Fire
Here is a list of the various levels of brightness.
1. Faint = Only just shows a play of color under direct sunlight.
2. Dull = Some color shown under low light, but still dull.
3. Bright = Fair color under low light and very nice fire under indirect light.
4. Very Bright = good color under low light and sharp crisp color under
5. Brilliant = Shows exceptionally bright crisp color under light.
The consistency or sameness in all the characteristics of an opal, including color, pattern, density of fire and color of the background.
Opal Pattern Variety
Although no two opals are the same, there are many excellent patterns of the diffracted colors and when these are combined with the right brilliance it can have a big impact on the price. Generally speaking the pinfire and small type patterns are not as desirable as the broad patterns or large flashes.
Some distinct patterns, such as rolling flash, straw pattern, Chinese writing, ribbon and harlequin are very rare and are collectors’ items. Almost all opal displays some form of pattern but you will never find two opals that are exactly the same.
It’s the amount of color and its intensity that makes one better than another.
In particular, many of the top quality pieces of black opal from Lightning Ridge show their strength through a distinct recognizable pattern play of strong colors, some of the various patterns are listed below.
Opal Harlequin Pattern
The opal harlequin pattern is the most prized of all patterns and the rarest. The colors in this pattern are usually arranged like checker board squares.
Opal Floral Pattern
The opal floral pattern is a very popular pattern and possibly the most common of all nice patterns Its name is derived from its likeness to floral dress material, this pattern covers a broad range of design and color.
Opal Pinfire Pattern
The opal pinfire pattern closely resembles a mass of pinheads in different colors. Another popular pattern.
Opal Palette Pattern
The opal palette pattern looks like an like an artist’s palette.
Opal Rolling Flash
The opal rolling flash usually consists of one color traveling across all or part of the stone. Noticed mostly when the stone is moved.
Rare, a transparent or semi-transparent opal is also known as a crystal opal.
The crystal opal is a solid transparent opal which shows a play of color but no base color. However this opal will show little color when on a white background unless the colors are very strong and vibrant in it.
Crystal Opals from Coober Pedy and Lighting Ridge, in Australia, are very rare and beautiful and command very high prices.
Thickness of the Color Bar in Opals
Some black opals and black boulder opals, with a thin color bar can be worth many thousands of dollars per carat. The thickness of bar sometimes enables a cabochon to be cut from the rough, thereby increasing value.
Opal Shape and Polish
The most common shape for an Opal is a domed oval, the length of which is around 40 percent greater than the width, and the height or dome equal to about 60 percent of the width. Of course, this is very difficult to find, as Opals are formed irregularly by nature and not by man.
Consequently, perfectly-shaped ovals will command a premium price as some opal has to be sacrificed when cutting and polishing. In fact over half the actual stone can be cut away to produce the finished opal
The quality of the work performed by the opal cutter, of course, can significantly affect the value of the Opal. It should be polished so that no scratches or imperfections are visible to the eye.
When inspecting an opal, it should be visually assessed for the face up clarity as well as any visible inclusions, patches of potch or lines, webbing scratching or any other imperfections. Any imperfections can seriously affect the value and therefore the price of the stone.
It is important to note that any crazing such as surface-reaching cracks and fractures, on any opal will render it worthless.
For the consumer it is all about eye appeal. opals are unlike diamonds or other gemstones and eye appeal can weight more heavily than many other considerations.
Some opals are described as having that elusive quality called character,
It is a fact that every opal presents different characteristic and is very important. Unlike other gemstones such as sapphires, rubies or emeralds for example, each opal will show its unique color combinations. It is very rare to see two similar stones, and impossible to find two which are identical.
Opals with more attractive and appealing color combinations have higher values, the color patterns of many Opals appear as though they could have been painted on.
Like diamonds and other gem stones, the smaller sized opals are worth less per carat than the larger stones. This is most particularly true with the black opals. As with any other gem stones, the larger the stone the more rare it will be and the more value per carat it will have.
There are a number of imperfections which can affect the value. Although sand, “cotton” and other impurities which are not clearly visible from the top of the stone will have little impact. Indeed, many collection quality opals will have sand in the back of the stones.
Cracks will drastically reduce value. A cracked or crazed opal has little, if any, commercial value unless the stone can be recut top remove the cracks or craze. Cracks can be distinguished from inclusions and vertical color bars by examining the opal with a strong light behind the gem.
When the stone is slowly tilted, the examiner should see a glint of light reflecting from the crack, (except where a crack is silt filled) sometimes it looks like a small brown fan inside, this is what is called a ginger whisker in Coober Pedy.
Another tip is to inspect the stone with a jewelers 10x loupe if a mark breaks the polished surface, then it is a crack.
Opals are a beautiful gem stone and, in the right setting, can look exceptionally beautiful. Having some understanding all about opals can make a big difference to getting a ordinary opal and an opal that will light your fire!