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Opal

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Opal Terminology

A rose by any other name would smell as sweet, but there would be a lot of confusion if some people called it a rose and others called it a dandelion. In opal terminology that is exactly what has been happening over the years. Various books and articles have been published since the 1960s which offer differing interpretations of opal terminology. The result has been confusion and, unfortunately, not a little misrepresentation.

My book, Opal Identification and Value (Majestic Press, 1992) hoped to eliminate this confusion by offering a universally accepted terminology for opal. To develop this terminology I interviewed numerous industry specialists from Australia, Hong Kong, Germany and the United States. I was quite surprised at the agreement among these specialists. It was easier than I thought it would be to generate an industry consensus on opal terminology.

Although Opal Identification and Value (OIV) has circulated widely in Australia as well as the rest of the world, the Australian gem industry felt that they needed to produce their own standard terminology. To meet this need a committee was formed which represented the major industry groups in Australia. After several years of debate a terminology was adopted. It has been published by the Australian Gem Industry Association (AGIA) as Opal Nomenclature and Classification, Xerox, Sydney, February 11, 1998. It is now the official terminology of the Australian gem industry.

I was quite encouraged to discover that this terminology differs from mine in only minor ways. There are no inconsistencies between the two. What differs are words and only a few of those. The result, I hope, is a consistent internationally accepted terminology with will eliminate the confusion in the industry.

Precious Opal

Opal is divided into two categories precious and common. Precious opal must have a play of color, sometimes called fire. Precious does not necessarily mean valuable. A stone can be classified as precious opal because of the play of color and still be junk. A common opal does not have a play of color but it still can be a valuable gem. The common gem opal you are familiar with is the faceted orange to cherry colored opal called “fire opal” in Mexico. There is a variety of common opal that is opaque with no particular pattern or beauty that is called potch.

Types of Natural Opal

The AGIA distinguishes three types of natural opal and varieties within each type differing in body tone (vase color in OIV), transparency (transparency or clarity in OIV and hue.

The AGIA refers to natural opal as “…opal which has not been treated or enhanced in any way other than by cutting and polishing. (AGIA, p. 1.) This means that any stone which has been treated chemically to alter its look, such as the treated matrix opal from Andamooka, Australia, cannot be classified by the AGIA criteria as natural opal. This should create some interesting discussions in the industry.

Solid Opal is called Natural Opal Type 1 by AGIA. It is simply a piece of opal “…in its natural state apart from cutting or polishing and is of substantially homogenous chemical composition.” (AGIA, p. 1.) This is opal you are most familiar with.

Boulder Opal is called Natural Opal Type 2 by AGIA who acknowledges that it is commonly known as boulder opal. It is “…one piece where the opal is naturally attached to the host rock in which it was formed and the host rock is of a different chemical composition.” (AGIA, p. 1.) The most familiar boulder opal is the material from Queensland, Australia where the opal forms in association with brown ironstone. However, there are other boulder opals with different host rocks.

Matrix Opal is called Natural Opal Type 3 by AGIA. It is “…opal presented in one piece where the opal is intimately diffused as in fillings of pores or holes or between grains of the host rock in which it was formed.” (AGIA, p.2.) AGIA says that it is commonly known as matrix opal.

Like boulder opal, there are a wide variety of host rocks that form matrix opal. The two matrix materials most evident in the market are the Andamooka matrix (usually treated and thus not natural opal by AGIA standards) and the boulder matrix from Yowah in Queensland, Australia. (See OIV, p. 17.)

Varieties of Natural Opal

Natural opals vary in their base color. Base Color is the combination of color (hue), body tone and transparency (clarity). A complete description of the stone requires mention of all three.

Hue is the body color of the opal apart from the play of color. Most Australian opals are white or colorless in hue. Opals from Mexico have an orange hue ranging from light yellow through orange to red. Opals from British Columbia, Ethiopia and some U.S. locations have similar hues. Other U.S. and Mexican locations have a blue hue, again in varying degrees of saturation and body tone. (See OIV, Chapter 5.) AGIA mentions hue but only in passing.

Body Tone “…refers to the relative darkness or lightness of the opal when ignoring its play-of-colour.” (AGIA, p. 2.) The AGIA is developing a body tone scale ranging from black through gray to white. The description implies that it will be almost identical to the Lightning Ridge Miners’ Association Tone Scale reproduced in OIV. (p. 49.) In OIV body tone is referred to as dark to light with black and semi-black being divisions within dark opal. The AGIA criteria divides tone into black, dark and light. In their terminology a dark opal (equivalent to semi-black) is lighter than a black opal. So the opal is dark but not as dark as a black. I find this terminology confusing and it is the only area where I take exception to the AGIA terms. It seems that it would be difficult to explain to a customer that a black opal is not a dark opal. In any case we agree on the division between the tone groups.

Black Opal has a black or very dark body tone when viewed face up. The criteria for how dark a stone must be to be called a black is set forth by the criteria reproduced in OIV and forthcoming by AGIA. Be aware that a dark black to the stone does not make it a black. It must show the dark body tone from the face.

Semi-black Opal has a medium gray body tone when viewed from the face. AGIA calls this dark opal. The criteria appears to be identical to that in OIV.

Light Opal has a white, clear, or light gray body tone when viewed from the face.

In addition to the hue and body tone, opals can differ in degree of clarity. Clarity is the degree to which light passes into and through a stone. It ranges from opaque to translucent to transparent. The industry agrees that a transparent opal is called a crystal opal even though it has no crystalline structure. A semi-crystal opal is translucent, that is, light passes through it but it has a hazy look (semi-transparent in AGIA terms). The AGIA calls both transparent and semi-transparent opal “crystal opal.” While I believe there is an important distinction to be made, the market recently seems to be treating the less transparent opal as in the same category as the clear opal so AGIA’s terminology reflects current market practice.

The degree of translucency necessary before an opal can be called a crystal or semi-crystal is a matter of judgment. The term opaque is not expressed but implied if a stone is not classified as crystal.

Composite Natural Opal

A composite opal is one where an opal has been glued onto another material. There are three main forms.

Doublet Opals are composite opal where a slice of opal has been glued to a backing or a clear material has been glued to the top of the opal. Backings range from common black opal potch to ironstone to black onyx or black jade.

Triplet Opals are a three-part composite with a thin slice of opal glued between a clear cap and a dark base.

Mosaic Opal is a composite of small chips of opal glued to a dark base.

Synthetic and Imitation Opal

In recent years more man-made opal is appearing on the market. A synthetic opal is of the same chemical characteristics as a natural opal but is grown in a laboratory. Recently synthetics have gotten so good that they are difficult to distinguish from natural opals. Imitation opals have the look of opals, more or less but are of a distinctly different chemical composition from natural opal. Common imitations are made of glass or plastic.

Disclosure

In Australia the law requires that each opal sold in the retail market have full disclosure in writing of the opal’s characteristics. In the U.S. there is no legal requirement to disclose yet, but I am sure it is coming. One industry group, the American Gem Trade Association (AGTA), does require all its member dealers to disclose any treatment of the gemstone.

Complete disclosure would require filling out the following list:

OPAL DISCLOSURE FORM

Type of Opal Precious/Common

Natural/Synthetic/Imitation/Treated

Solid/Boulder/Matrix/Composite (doublet/triplet/mosaic)

Variety of Opal

Hue (clear, white, orange, blue, gray, etc.)

Body Tone (light, semi-black, black)

Clarity (opaque, translucent [semi-crystal], transparent)

Treatment

Untreated/Dyed/Stabilized

Physical Description

Weight (carats)

Dimensions (length, width, depth in millimeters)

Shape (oval, pear, etc.)

Cut (cabochon, faceted)

Origin (if known)

Full disclosure assists the purchaser in several ways. It notifies him or her of what has been purchased. It guarantees that the opal has been represented correctly if the disclosure refers to an industry-wide standard as is the case in Australia. It also offers evidence should the stone be misrepresented. Additionally it assists the dealer in that it adds confidence for the purchaser and assures that the purchaser cannot come back later and claim the dealer sold them a black opal and delivered a doublet when the invoice clearly discloses the stone as a doublet. There is nothing I would like better than to have complete disclosure required by U.S. law.

So, there we have it. With this standardized opal terminology we can all agree to call a rose a rose. And with disclosure the purchaser and dealer will be assured that it is indeed a rose.