How is the Value of Opals Determined?
An opal’s worth is a difficult thing to measure. The type of opal, body tone, brilliance, pattern, color bar thickness, play of color, and faults all play important roles in determining the value. Other important variables to consider include the quality of the cut and polish, and the size of the stone. When being valued, an opal is carefully examined and given a price per carat. The overall carat size of the stone will then determine the price of the opal.
It is essential to first identify the type of opal that is being valued. An opal doublet or triplet can be worth considerably less than a solid opal. Doublets and triplets are 'assembled' stones that only contain a very thin slice of natural opal and are therefore generally much less valuable.
Body tone is one of the most important factors in the classification and valuation of opals. It refers to the background or the underlying color of the opal, which ranges from dark to light. Generally, opals with a black or dark body tone are more valuable than those with a white, light, or crystal body tone, because a stone with a darker body tone tends to display colors more vibrantly.
Black opal is the most prized type of opal and may earn prices over $10,000 USD a carat. Boulder opals also have a dark body tone but are not quite as valuable, while white opals have a light body tone and are generally the least valuable form of opal.
The term crystal opal refers to the 'diaphaneity' (transparency) of an opal, not its crystal structure, and is defined as any type of opal that is translucent or transparent. Some crystal opals display color so intense and dark that they are referred to as black crystal opals.
Play of Color
The phenomenon known as the play of color is the primary attraction of the opal. It is caused by the diffraction of white light across the internal structure of orderly arrayed spheres of silica, and produces the brilliant spectrum of colors the opal is known for. Red (fire) opal is generally more valuable than a mainly green opal, which in turn is more valuable than a stone showing only blue color. Nature does not produce a red color as often as it does a blue or green. Red coloring is caused by larger, rarer constructions of microscopic silica spheres, whereas blue is caused by the more common small spheres.
Brilliance refers to the brightness and clarity of the colors displayed by an opal, when the stone is viewed face-up. This ranges from brilliant and bright to subdued and dull.
The pattern of colored segments that form the play of color of a precious opal is unique to each individual opal. The distinctiveness and color displayed by these segments determines the quality of an opal’s pattern.
Excellent patterns include:
Harlequin - Large sections of color in which each color segment is roughly the same size and shape, like a mosaic or checkerboard. A true harlequin pattern is extremely rare and highly sought after.
Flagstone - Large sections of color with straight edges, in a random pattern.
Ribbon - Narrow, parallel cascading lines of rolling color.
Straw - Random thin strips of overlapping color.
Chinese Writing - Thin strips of overlapping color which resemble Chinese characters.
Picture stones - The intriguingly unique patterns of 'novelty' or 'picture' stones, which resemble an object, landscape, animal, person, etc.
Good patterns include:
Floral - A random pattern of color with good spread.
Rolling Flash - Large sections of color which roll across the stone as it turns.
Broad Flash - Large sections of color which flash as the stone turns.
Pinfire - Tiny points or specks of color.
The thickness of the color bar in opals is relative to the overall size and shape of the individual stone. Boulder opal, for example, typically has a very thin color bar due to the way the opal is geologically formed. While this should be taken into account when valuing the stone, it makes little difference to its appearance once set in jewellery.
Faults that can detract from the value of a finished opal are many and varied. A crack in the face can render worthless an opal that otherwise might have been worth a considerable amount. Crazing (many small cracks in the opal's face) will also relegate the stone to worthlessness. Sand and various other minerals can be found as inclusions in and/or under the color bar, and in the potch of opals. Other faults include potch lines, webbing (grey lines), and windows (sections devoid or lacking in color). The consistency of colors and pattern when viewed from different directions also has an influence on the value; an issue arises when a stone won't face, a problem in which the color only shows through on certain angles and otherwise has little color. The visibility of potch or brown ironstone on the surface of the stone will also lead to a drop in value.
The term base color is often confused, but is very simple in actuality. Think about looking at an opal and extinguishing all the fire colors from it; what remains is the base color of the stone. Base colors, sometimes called background colors, include black, semi-black, crystal, semi-crystal, white, gray, blue, orange, brown, boulder black and boulder brown. Base color is one of the four primary characteristics that determine an opal's value.
The base color of an opal is determined by looking directly at the top of the stone. With your eye above the stone, look down at the face of the opal as it sits on a flat surface. You will notice that it has two color characteristics: the play of color and a general base color. It is the general base color we wish to determine here.
Base color is actually a combination of three things: color or hue, body tone or saturation, and transparency or clarity. A complete description of a stone requires mention of all three and each affects the stone's value.
A natural solid opal that is opaque when viewed from the top of the stone, and which has a play of color against a dark background and a grade of at least #3 Black on the Lightning Ridge Miners Association Tone Scale, is graded as a black opal. The back of the stone may be any color.
A natural solid opal that is translucent to transparent with play of color which, when viewed from the top, is given a grade of at least #3 Black on the Lightning Ridge Miners Association Tone Scale.
A solid opal that is translucent to opaque when viewed from the top and has a play of color against a dark gray background that is classified as semi-black on the Lightning Ridge Miners Association Tone Scale.
A natural boulder opal that has a play of color against a dark opaque background and is graded as at least #3 Black on the Lightning Ridge Miners Association Tone Scale.
A solid opal that is transparent and shows a play of color, but no base color.
A solid opal that is translucent and shows a play of color with a clear base color.
A solid opal that is opaque or translucent and shows a play of color on a gray base, corresponding to the gray tones on the Lightning Ridge Miners Association Tone Scale.
A solid opal that is opaque to translucent and shows a play of color on a white to off-white base color.
An opal graded as crystal, semi-crystal, white or gray that does not fall into a more specific category.
A solid opal that is translucent to opaque and shows a play of color on an orange background. This type of opal is called red opal or yellow opal, depending on the base color.
Orange Crystal Opal
A solid opal that is transparent and shows a play of color on an orange background. This type of opal is called red crystal or yellow crystal depending on the base color. This is generally referred to as fire opal.
A solid opal that is transparent and shows a play of color on a brown base color.
A solid opal that is transparent and shows no play of color, but may show an opalescence without a pattern. The orange material without a play of color from Mexico is frequently seen in faceted stones, and would be called orange jelly, although it is also called fire opal in the industry. The blue jelly from Australia is sometimes called blue bottle potch.
A solid opal that can be translucent to opaque with an optional play of color on a blue base color. The opaque blue common opal from the Andes is just called blue opal. There is also a pink variety of this common opal.
Opals being mesasured on the brightness scale.
Opals being mesasured on the brightness scale.
A collection of our light opals.
Opals being inspected for body tone.
Opals still in the preparation stages attached to dop sticks.