Identifying Opal Doublets

(via Gemological Digest, Vol.2, No.4, 1989) Grahame Brown writes:

Abstract
Opal doublets have in the past been produced mainly to utilize material otherwise too thin for cutting solids. They were not normally produced for fraudulent purposes, and identification was simple, due to an even join between opal and backing. Today, a new and more sinister type of opal doublet has appeared. Produced by bonding a rock backing to an irregular piece of precious opal, the result is a stone which looks very much like Queensland boulder opal, even to the trained eye.

In this article, the author discusses both the history of opal doublets, as well as the latest incarnations, and describes useful techniques for recognizing these frauds.

Introduction
The opal doublet is a cabochon-cut composite stone. It consists of two components: a domed ro flattened top of precious opal and a base of dark colored, light-absorbing material.

While natural opal doublets can be cut from some precious opal that occurs as near parallel layers of either opal potch, opal-ironstone, -sandstone, or –mudstone, most commercially marketed opal doublets are manufactured products that have been deliberately assembled, preformed and polished by man.

Origin
Available evidence suggests that the assembled opal doublet of Australian origin. It was first devised and manufactured in 1897 by miner-lapidaries on the remote, arid north-west New South Wales opal field of White Cliffs, 1000 km north-west of Sydney. The reason why these 19th century opal miners chose to manufacture the opal doublet was one of simple economics: only about 5 percent of the production output at White Cliffs was gem quality white opal. So, a use had to be found for the many thin veins of precious opal that could not be used for cutting solid opal cabochons. The opal doublet, made by cementing a thin layer of precious opal to a backing of dark grey opaque potch, was the simple, yet economic, answer to the low yield of precious solid opal from the White Cliffs field.

Method of manufacture
Today, opal doublets are commercially made for at least three reasons. First, the mined precious opal may be too thin to cut a solid opal. Second, the opal may have such a pale body color that its play-of-color (and value) could be enhanced by backing it with a suitable dark colored light absorbing material. Finally, by adhering a suitable backing to a thin layer of opal, the resulting composite could then be used to imitate opal of much greater value and rarity.

Choice of opal
Opals suitable for doublet manufacture should display the following characteristics:

- Freedom from clearly visible flaws (cracks, crazing, webbing, sand or gypsum inclusions, unsightly potch bars).

- A reasonable transparency, allowing light to penetrate through the opal top to the light absorbing backing.

- A distinct and rather muted play-of-color. This play-of-color is enhanced by being strongly contrasted against the dark backing.

- Sufficient thickness of precious opal to allow a flat plane-of-attachment to exist between the precious opal top and the light absorbing backing.

Consequently, flatish fragments of white or other pale-colored jelly or crystal opal make the most striking opal doublets. The use of opaque whitish to grayish opal only creates dull, lifeless doublets that display a poor play-of-color.

Choice of backing
The best backing for opal doublets is opaque black potch from Lightning Ridge. Due to the rarity of Lightning Ridge black potch, other opaque black backings, such as black glass, black plastic, obsidian, and even black onyx have been used to produce opal doublets imitating black opal. Care should be taken when choosing black backings for opal doublets: a backing with a coefficient-of-expansion differing from that of precious opal (e.g. black onyx) could fracture the doublet if moderate heat is applied to the finished composite.

With increased demand for dark Queensland boulder opal as an alternative for the increasingly rare and valuable Lightning Ridge black opal, manufacturers have begun producing opal doublets intended to imitate Queensland boulder opal. The precious opal component of these doublets may consist of thin, flat segments of Queensland boulder opal, whitish crystal opal shell pseudomorphs from Andamooka (South Australia), translucent to transparent whitish opal from Coober Pedy, Mintabie (South Australia) or White Cliffs (NSW). Backing for these doublets includes ironstone, dark-colored sandstone, and mudstone, commonly associated with Queensland boulder opal. There exists also a composite backing made by incorporating crushed fragments of the above materials into a black pigmented epoxy resin plastic.

A black backing is unnecessary when a doublet is made only to provide thickness and strength to a thin sliver of precious opal displaying a valuable color pattern. In such cases, grey potch, plastic or glass usually suffices as suitable backing.

Choice of adhesives
To permanently affix the top and backing of an opal doublet, most manufacturers use an epoxy-resin adhesive that is pigmented by carbon. Setting times of these adhesives vary from 5 minutes to 24 hours.

Methods of identification
Conventional opal doublets are identified traditionally by observing the presence of:

- Three components: a top of precious, natural opal; a backing of dark colored potch opal, glass, plastic, ironstone, sandstone, mudstone or iron stone filled black epoxy resin; a thin layer of adhesive permanently joining the top to the backing of the cabochon.

- A flat, junction-plane between the precious opal cabochon top and backing. This junction can vary in thickness. It often has gas bubble inclusions entrapped in the adhesive.

- Included, somewhat squashed, bubbles within the junction-plane of the doublet. The visibility of these bubbles is increased when the epoxy resin is viewed through the transparent opal top with the aid of an intense fiber optic light source.

- Occasional evidence of a pigment layer used to darken the flat-ground undersurface of the precious opal top of the doublet.

- Evidence of lifting of the top of the doublet from its backing, if the components are poorly adhered.

It is more difficult to identify some of the recently manufactured doublets designed to imitate boulder opal. These imitations are characterized by:

- Very thin, non-planar junctions between the top and the backing of the doublet.

- Junctions that are relatively free of included bubbles within the epoxy resin.

- Backings that are cut from coarse-grained sandy Quilpie ironstone/sandstone, or dark fine-grained banded Winton mudstone. Some of these backings are also included by thin anastamosing veins of precious opal.

To differentiate these effective look-alikes from the natural Queensland boulder opal they imitate, gemologists must be prepared to examine closely the junction separating the two components of a suspect doublet. In particular, one should look for identifying visual characteristics of Queensland boulder opal, such as:

- An irregular epoxy resin-free junction between the precious opal top and the ironstone, sandstone, or mudstone base of the cabochon. No visual evidence of an adhesive bubble-included junction should be seen when this interface is examined through the natural top of the boulder opal cabochon. Fiber-optic illumination greatly assists a detailed examination of the junction of most opal doublets.

- Tongue-like projections of precious opal that penetrate the sedimentary rock base (matrix) of the cabochon.

- A cabochon base formed from dark colored sedimentary rock (sandstone, ironstone, mudstone) that does not contain black plastic matrix or included gas bubbles.

Conclusions
It is not difficult to identify conventional opal doublets, made by cementing with black epoxy resin a top of diaphanous light colored jelly or crystal opal to a backing of dark potch, glass, plastic, or sedimentary rock. A simple 10x hand lens should quickly disclose the tell-tale, uniformly thick planar junction that is a feature of these composite stones. Hand lens or low power microscopic examination of the junction of conventional opal doublets should reveal gas bubble inclusions and/or solid particulate fillers within its content of black epoxy resin adhesive.

It is somewhat challenging to distinguish cabochons of Queensland boulder opal from their newer-manufactured doublet imitations. This is because the adhesive-filled junction between the precious opal top and the ironstone, sandstone or mudstone backing of these doublets is extremely thin, highly irregular in profile, and is made much less conspicuous by incorporation of finely ground particulate matter in the black epoxy-resin adhesive filling the junction. Careful low power microscopic examination of the junction area is required to identify the distinctive features of this effective look-alike of Queensland boulder opal.