In common opal the spheres are of various sizes and arranged in an irregular pattern. This irregular pattern scatters much of the light that enters the opal. However, in precious opal the spheres are all the same size and precisely arranged in a regular, repeating network. When the spheres in this network are exactly the right size and in exactly the right arrangement, they will diffract the passing light into its component colors.
As the light exits the opal, it displays the play-of-color that gives precious opal its unique beauty. Precious opal must be cut with much care, skill and study. Before cutting, the piece must be carefully examined to identify the location, orientation, and pattern of its play-of-color features. The goal is to produce a stone of maximum play-of-color, uniformly distributed, and in a shape that is well suited for jewelry or other use.
The best precious opal displays a full spectrum of bright colors, uniformly distributed across the face of the gem with no dead spots, and produces good play-of-color when viewed from any angle. It is extremely rare to encounter a gem that meets all of these goals.
These special gems are what the miner hopes to find and the cutter hopes to render. Precious opal that meets these requirements is especially valuable. Boulder Opal and Matrix Opal: The stone on the left is a bead cut from boulder opal. The cabochon on the right is an example of matrix opal. Both of these materials were mined in Australia. Boulder opal and matrix opal are two other categories of opal that contain the play-of-color known as "precious opal".
The name " boulder opal " is used for a rough or cut gemstone that exhibits play-of-color originating from precious opal in seams and patches within the host rock. The name " matrix opal " is used for a rough or cut opal in which the precious play-of-color is intimately mixed within the parent rock.
It is often within the interstital spaces of a sedimentary host rock or within the vesicular spaces of an extrusive igneous rock. Solid Opal and Assembled Stones: Some opal is cut from solid pieces of rough, but when only small or thin pieces of rough are available, assembled stones are sometimes made. An "opal doublet" consists of a thin slice of precious opal glued to a solid base often made of black obsidian. This also allows play of colour to be observed from the side of the stone.
The factors above are very important in valuing opal and should all be taken into consideration when coming to your conclusion. In my opinion, the factors considered the most important when valuing opal in the commercial gem trade are the dominant colours seen with regard to play of colour and how extensive the play of colour is across the stone.
This includes the variety of patterns that may sometimes be seen within the play of colour. However, I believe, as I have stated, that some of the factors above should be taken into greater consideration, and I will explain why as we discuss the last factors.
As specified at the beginning, opal is an amorphous hydro-silicate. This means opal has no repeating crystal lattice and therefore no crystal symmetry and is composed of silicon and oxygen with a percentage of water within its structure. Sometimes in opal the silica spheres aggregate together in an orderly fashion, arranging themselves into a symmetrical repeating pattern within the amorphous structure.
The spheres will range in size and abundance. The aggregation of these orientated silica spheres is the reason we see play of colour in opal.
Play of colour occurs because the light entering the opal will pass through the gaps between the silica spheres. When the light passes through it is forced to bend to fit through the gap; this bending effect splits the light into the colours of the spectrum.
An unusual and highly sought-after black opal with green play of colour in a Chinese writing pattern. The colour seen is dependent on the size of the gap between the spheres, which directly correlates to the size of the spheres themselves, which are only a few hundred nanometres in size. Sometimes the gaps are so small only violet light can pass. Larger gaps are uncommon, therefore reds and oranges as the dominant colours are rarer than blues and violets; so, all other factors being equal, they are more valuable.
In particular with black opal or dark-toned boulder opal the reds and oranges have an even better contrast. Consequently, red as the dominant colour on a deep-toned black opal will be the most valuable. We say dominant colour because it is quite usual to see other colours within the stone too, especially when the reds and oranges are present. To just see red or orange without the yellows, greens and blues is very rare. As I stated above, other factors need to be taken into greater consideration by the trade and brilliance or brightness is definitely one of them.
It is such an important factor. Patterns and how play of colour is distributed across opal are the final factors to discuss. In order to assess these value factors, one should ask, is the play of colour spread across the entire stone or localised to one place?
In fine quality opal not displaying patterns, you would hope to see the play of colour evenly spread across the surface, albeit at different intervals. Commercial opal may only have patches showing play of colour. If so, the position of this play of colour becomes a sub-factor. Value will also drop if the best view of the play of colour is from the side. Much like a prism, which can refract white light and produce a rainbow effect, opals diffract the white light which is coming from above, displaying those amazing opal colours.
Basically, opal is made up of water and silica the main component in glass. A silica solution forms when silica from under the earth mixes with water. This solution fills voids or is trapped in layers under the earth, and opal begins to form. Learn more about how opal is formed. Over a long period of time, the solution settles and the water evaporates, allowing the gradual formation of layer upon layer of microscopic silica spheres.
The spheres are formed because particles of silica spontaneously adhere to other particles which form around it. These spheres of range in size from to angstroms 1 angstrom is 1 ten millionth of 1 millimetre. Because they are spherical, there are tiny gaps remaining between the spheres much the same as when marbles are placed together in a container. In these gaps between the stacked spheres, a water and silica solution remains.
The spheres in an opal are not only remarkably uniform in size but are packed, in gem quality opal, in a very regular array. See image, left — An electron-microscope photograph of of the ordered structure in precious opal, showing its light-diffracting spheres. When white light waves enter the top of an opal, they refract and bounce around inside the opal, through all the microscopic spheres and the gaps between the spheres.
As the light passes through the spheres and gaps, it diffracts splits. Like a prism, the opal splits the white light into all the colours of the spectrum, and the light eventually bounces back out the top of the stone, at which point we get an eyeful of beautiful opal colours. The opal is the only known gemstone that is able to naturally diffract light in this way.
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