Black Opal Brightness and Pattern Charts
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Black Opal Rare Patterns
Black Opal Harlequin Pattern
Black Opal Grading Info
Preservation and Handling of Opals
Opal is a "living" stone, which means it must be protected from heat and detergents that "dry" the gem.
Opals develop crazing if they are allowed to dry out.
Heat treatment is catastrophic!!
In addition to cracking, loss of water causes loss of irridescence.
How Opal Colour is Produced
It took the development of the electron microscope to work this out. Precious opal is made up of tiny uniform spheres of
transparent hard silica, which fit together in an orderly three-dimensional frame, sitting in a "bath" of silica solution. It is the orderliness of the spheres that separates precious opal from common opal. Light passes through the transparent spheres in a direct line, but when it hits the 'bath' of silica, it is bent and deflected at different angles, thus producing a rainbow effect.
Deflection & Diffraction
Depending on the size of the spheres, varying colours of the spectrum are diffracted. So it is a combination of deflection (bending) and diffraction (breaking up) of light rays that creates the colour in opal. If you move the stone, light hits the spheres from different angles and brings about a change in colour. The name opal actually means "to see a change in colour." The way in which colours change within a particular stone as it is rotated and tilted is called the stone's play of colour.
How colour is defined
The size of the spheres has a bearing on the colour produced. Smaller spheres bring out the blues, from one end of the
spectrum. Larger spheres produce the reds from the other end. The more uniform the spheres are placed, the more intense, brilliant and defined the colour will be.
Buying Opals
The last couple of years have been very difficult for the opal industry because of the running out of traditional fields and the failure to find new ones. You will be pleased to know that we have formed a partnership with a number of miners who have found some new fields and have come through with some new stock. Hopefully this will continue.
Solid Opals. A solid opal, as the name implies, is a single piece of opal. Solids may be cut into a variety of shapes ranging from circular hemispheres and ovals to free-flowing shapes that follow the natural lines of the stone.
Doublet Opals. Doublets are created by taking a thin sliver of natural crystal opal, then attaching it with jeweller’s cement to a natural black opal backing (The backing is called black potch and lacks the play of colour characteristic of opal) or black onyx
Triplet Opals. A triplet is also a composite piece. To create a triplet, the jeweller takes a sliver of natural crystal opal, then attaches it with jewellers cement to a backing of black potch or black onyx. A cap of clear quartz is then placed on top of the opal sliver, creating, if you will, a sandwich effect.
Opal Shapes
Opals can be cut into a wide range of popular shapes. Oval is by far the most popular shape, followed by the circular or round shape. Teardrop, square, rectangle, and triangle are other common shapes.
Some opal cutters prefer to let the opal define its shape rather than imposing a shape on the stone. Opals cut in this manner assume unusual freeform shapes.
Most solid opals and doublets are cut and polished with a convex dome or curved top. This encourages light into the stone so that the opal's iridescent properties are shown at their best. For triplets, the dome is created by a crystal cap, which covers the flat slice of opal.
Australian opal is rarely faceted, nearly always domed. A domed stone is stronger and less prone to chipping. However, opal without a play of colour (such as cherry red or amber opal from Mexico) is often faceted and can look a lot like ruby.
Opals are a carver’s delight. The stone is not too hard to work with and will not wear out your diamond tools as much as such stones as chrysoprase, agate, or the like.
Opal Size
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Popular sizes are listed below. Sizes in bold type are more commonly available than other sizes.
Round: 5 mm and 6 mm
Oval: 6 x 4, 7 x 5, 8 x 6, 9 x 7, 10 x 8, 12 x 10, 14 x 10, 16 x 12, 18 x 13, 20 x 15, 25 x 18, 30 x 22, 40 x 30.
Opal Settings
Jewellery castings for opals can range from very simple designs to very ornate designs with accent diamonds.
There are some interesting myths associated with opal settings. Some folks believe that doublets and triplets are always put into fully backed settings to hide the potch backing. Others believe that full backings are used to protect the stone. Neither myth is true. A jeweller could just as easily "hide" a doublet in an open backed setting, as the join line between the stone and the potch backing would be covered by the side of the setting, making it difficult to see whether the back of the stone was a different stone or simply the less brilliant side of the same stone.
How Opal is Graded and Sold
Rough opal parcels are sorted into three grades: tops, middle, and low. Each tops parcel has a King stone, which is the best stone in the parcel. Some parcels have several King stones. Colour is the primary criteria for grading, but the graders also take into consideration the number of imperfections and faults, and whether a stone is the right shape to be cut into an oval or one of the other popular shapes. You can buy rough opal in several different conditions.
Mine Run. Direct from the mine. The stones have not been cut or ground down. This means that there is more guesswork in the cutting. Purchasing mine run opal can be risky if you are not very experienced.
Off - Cuts. The miner has removed whatever opal he has a market for and sells you what's left. With off cuts, you can usually tell what you are going to be able to cut. You must still watch for cracks in the opal because once a crack becomes obvious, a stone can lose half its value.
Rubs. This can often be the best way of buying rough if you are not very experienced. The miner has cut and ground the stones into basic shapes, after having removed most of the rubbish. What you have left is the stone nearly ready for the dopping and polishing process. You have the satisfaction of cutting your own stones without the high risk of buying mine run rough.
Opal is officially sold in troy oz. There are approximately 30 grams to a troy oz and approximately 30-oz to a kilo.
Hints and Tips
Here's an eclectic assortment of hints and tips you may find helpful when buying or working with opal.
1. Not all opal is valuable. Potch, is sometimes called "common opal" does not reflect colours. Potch does have its use,
however, as a backing for doublets and triplets. When there is colour mixed with potch it's called "potch and colour."
2. Some classes of rough opal are unsuitable for making gemstones. Some are very porous. Others have a chalky appearance. Still others can lose their colour in a short period of time. And yes, it is common for certain types of opals to shatter in a dry atmosphere when the water from the gel evaporates. The best protection for buyers is to purchase opal from a knowledgeable and reputable dealer who accurately details exactly what you are purchasing and is willing to guarantee the stones. Note: If you buy rough opal, there is no guarantee that it will not crack. This is the calculated risk you take and is part of the "excitement" of opal cutting.
3. When examining opal jewellery, look at the stone from the side to determine whether it's a triplet. Solid and doublet opal domes are simply the opals formed into a curved dome shape. Triplets have clear crystal domes to protect the flat opal surface.
4. It is very difficult to tell whether a stone is a doublet or a solid if it is in a setting. It helps if the back of the setting is open, but it is still no real indication because the back of a doublet and a solid looks very similar, although a doublet is usually
perfectly black on the back whereas a solid black opal usually has some imperfections. So if the back of a stone looks too
perfect, you have to be a little suspicious. For this reason it’s best to buy opal from reputable dealers who know what they are doing because, unfortunately, many jewellers also are not experienced in opal.
5. When buying opal over the net, ask the seller to give you a summary of how close the graphic sample is to the real thing. Opal is very difficult to photograph and the appearance on the computer screen may give it a different look. Particularly is this the case in regards to size. Graphics come out in all different sizes, usually a lot larger than actual...so make sure you get your ruler out and actually draw the size on a piece of paper so that you will know in your own mind’s eye the actual size of the stone.
6. The same thing applies to the colour of the stone. Make sure you get the supplier to give a commentary on the colour so that you will know pretty well what you are getting. The camera sometimes picks up colours that are only minimal, while ignoring the feature colours.
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Last Update: 30 Dec 2010