More than almost any other gemstone, opals are uniquely individual. Not only do they occur in a wide variety of types, but they can exhibit all the colors in the rainbow, often all at once, in flashes of color as the stone is rotated. This phenomenon, know as ‘play of color,’ makes opals one of the trickiest gems to evaluate.
From precious black opals with dark blue, green, or black backgrounds to the translucent deep orange fire opals of Mexico, opals can vary as widely as snowflakes, and are deeply treasured for the very variety that can make them difficult to judge.
Evaluating Color in Opals
Color is the most important factor when evaluating opals, and is by far the most complex aspect to judge. Opals occur in hues of every color, and often more than one at once. Body-color, also known as background color, is a result of suspended impurities in the opal, and it determines the type of opal, like black or white. Black is generally a more favored background color as the dark background tends to highlight any play of color.
Play of color is the defining characteristic of a precious opal’s appearance. Also known as opalescence, play of color is the combination of color flashes that occur when viewed from different angles. Regardless of the combinations of colors, play of color is most prized when it is vivid. An opal with bright play of color is always more valuable than one with faint play of color.
In addition to the brightness of an opal’s play of color, the range of colors is important as well. Some opals exhibit only one main color with one or two secondary colors, but opals with a play of color that spans the entire spectrum are highly valued, especially if the colors are vivid.
Opals with red as the prominent hue in play of color are traditionally the most valuable, followed by orange and green, though personal taste can also dictate value. Play of color can also change with the viewing angle, with certain colors dominant only from certain angles. The most valuable opals exhibit play of color when viewed from any angle.
Pattern in an Opal’s Play of Color
Pattern in an opal refers to the arrangement of the stone’s play of color, and can take many forms. Generally speaking, large areas of color that are closely assembled are preferred over small pinpoints or scattered areas. Regardless of the pattern, the colors must be vivid for the stone to be valuable.
Also of importance when considering the pattern is the inclusion of “dead spots” or what is known as extinction. These areas in which there is no play of color and only the stone’s background color is visible can greatly detract from the overall value of the opal.
Evaluating Opal Transparency and Clarity
Opals naturally occur in a wide range of clarities, from totally opaque to completely transparent. Different types of opals are valued for different transparencies, based mostly on how the opacity highlights the play of color. In crystal opals, transparency tends to accentuate the play of color, whereas in black opals, an opaque background better displays its play of color. Cloudy or milky background colors will generally lower the value of an opal, regardless of its type or color.
Like all gemstones, opals can exhibit fractures, pits, or other surface imperfections that can lower their value. Fragments of the ore surrounding an opal, called matrix, are sometimes included in the polished gemstone, and can also negatively affect the value. Crazing, or small cracks that look like a spider’s web, can occur in some opals when exposed to extreme heat or dryness, and also lower the value of the gem.
How Opal Cut Affects Value
Because of the delicate interplay of background color and play of color, opals are usually cut in a way that best accentuates the play of color. In particularly large and exceptional opals this can lead to asymmetrical shapes that are best suited for a custom, one-of-a-kind setting. Most opals are fashioned into symmetrical ovals, often en cabochon, as the domed surfaces tend to highlight play of color the best.
Opals are relatively soft and fragile, and as a result, thinner opal material is sometimes fashioned with backings to give them greater strength and durability. Assembled opals called ‘doublets’ consist of opal material with glass, obsidian, or plastic backings. ‘Triplets’ are a thin layer of opal cemented between colorless quartz on top and a doublet style backing. While still of value as jewelry, assembled opals are valued at only a fraction of solid opals.
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