The purpose of this guide is to give the reader a basic working knowledge of how to assess the value of opals and be a more informed buyer on eBay. For a more information, I would recommend the following: OPAL IDENTIFICATION AND VALUE - by Paul B. Downing, PhD.
First, let me tell you that I have been cutting opal for about 20 years. I fell in love with the stone during a trip to Australia in 1985 and began cutting opal as a result. In addition, I have set opals in both standard settings and done custom jewelry.
The most important point to make is that it is almost impossible to determine the value of an opal simply by looking at the pictures on eBay. This means that you must look for a dealer that is willing to let you buy/return the stone or be well enough informed to ask the dealer for information based upon a standard view of grading. When I value an opal, I use the method developed by Paul B. Downing and it is the most objective approach that I have seen so far. If you buy opals on a regular basis, it is worth a detailed read of his book. I will try to give you the basics in this guide.
The first point of consideration is the type of opal:
Natural Opal is opal that has not been modified in any fashion. It is mined, cut and polished.
Treated Opal is opal that has undergone a process to improve its characteristics. Typically, this is to either enhance its color or fill a crack.
Synthetic Opal is opal that is man made. It has the same chemical composition as natural opal, however, it is laboratory grown.
Simulated Opal does not have the chemical composition of opal. It is something that is made to look like opal.
When stones are sold on eBay, they are usually described based upon whether they are solid or a laminate. Solid stones are cut from rough and the stone is shaped and polished from the original rough. Laminated stones are stones that are made of a layer of opal and layers of some other material. A doublet has a layer of opal on top of a layer of opal potch, iron stone or black jade. This is done in cases where the stone is too thin to cut a solid or the colors show better with a dark background. Doublets can be extremely attractive and I have cut many of them. They are just worth far less than the comparable solid stone. A triplet is a stone that sandwiches a thin slice of opal between a backing (as in the doublet) and a quartz cap. This process uses much less opal and the magnification of the quartz cap can provide a brighter look. A triplet is worth less (typically) than a doublet. The following picture shows the construction of an opal triplet.
In general, a doublet is worth one tenth to one twentieth of the value of a comparable solid opal. Triplet pricing is based upon standard tables of commercial grades and sizes. Some handmade triplets can be worth considerably more than the commercial fabricated triplets.
For solid opals, the most important considerations are the type of opal, the base color, the weight of the opal and the brightness/pattern of the fire.
An opal, that has a crack is considered worthless from a commercial standpoint, as the crack will render the stone unstable for jewelry use. The crack will likely extend through the stone and fracture the stone over time and use. So, if you receive a stone with a crack, it should be returned immediately to the dealer. This can be a bit of a tricky determination as color line breaks often resemble cracks. So, unless you are practiced in looking for cracks, it is going to come down to dealing with a reputable dealer. (I can give some instructions for looking for cracks in a subsequent guide, if there is a positive response to this one).
The type of opal is based upon the considerations previously discussed. For the sake of this guide, let's consider solid natural opal as that is the most frequently advertised opal on eBay. It is useful to know the region that the opal was mined from. Some regions (such as Andamooka) produce opal that is commonly treated. Andamooka opal is white when it is mined, but it is treated with a dye to make it look black.
Base Color - The base color is the background color that the stone presents as viewed from the top. The color of the back of the stone is not consequential to the value of the stone. The most valuable base color is black. Base colors can be white, clear, orange (typically mexican opal), gray, semi-black etc. The Lightning Ridge Miners Association has produced a a standard tone scale to judge base colors.
In general, black is worth much more than a white or crystal opal. So, when buying, it is critical that a stone sold as black meets the standard criteria for body tone.
Brightness of Fire - the fire is the play of color in the stone. It can be single solid colors (red, green, blue, orange) or mixtures. Some stones have composites of color comprised as many dots and others have color that occurs in sheets. Rare stones will have color patterns that are valued for their specific patterns. The type of fire and brightness are critical in determining the value of the stone. Paul Downing established a brightness scale that ranges from 1 (faint) to 5 (brilliant) that can be used to describe the overall brilliance of the stone. This must be done with correct lighting as the impression of the stone will vary significantly with the lighting. Many dealers on eBay are versed with this scale and can describe the brightness of the stone as measured in this standard fashion. It is important to realize that opal is often not uniform and may have different areas of brightness or directionality. So it is important to ask the dealer if the stone is uniform and whether there is any directionality to the brightness.
The following picture gives you an example of the varying brightness of stones. The stone on the left was catagorized as a brightness 2 stone and the stone on the right was catagorized as a brightness 4 stone. Do not use this picture as a brightness guide as it is just meant to give a relative feel.
Color - Every opal is unique and that is what makes it such an interesting stone. However, there is typically a dominant color and a generalized pattern in the stone. Typically a stone will be described as a single color. That means that this is the only color in the stone. Stones that are multicolor have two or more colors, but there is always one color that is dominant. A Blue-Green stone shows a color dominance toward Blue and a Green-Blue stone shows a color dominance toward Green. Most dealers would maintain that to really be multicolor a stone it must have three colors. A true multicolor stone is more valuable than a stone with a single or two colors. In general, red is more valuable than any other color, but a multicolor stone is more valuable than a single color stone. So a red multicolor on a jet black background is the most valuable provided it is cut well and has no flaws. Confused? The variants that effect opal valuation are confusing and that is why it is critical to understand them. Otherwise, it is impossible to know whether you are paying a fair price for a stone or not. AND THERE ARE THOUSANDS OF STONES ON EBAY!
If you find this guide useful, please indicate by voting positively. If so, I can write another chapter covering the details of cut, quality, and patterns of fire. Hopefully this guide has made you a more informed eBayer.