Synthetics & Imitation Opals
In the Lightning Ridge and Grawin Opal Fields, two variations of precious opal formed simultaneously:
Synthetic opals are often called Gilson Opal due to the pioneer in this field. At a glance they look like natural opal but as the opal is rotated, the play of colour does not change (remains very static, two dimensional). Viewed from the side the colour segments tend to look like columns.
There is typically no potch associated with the opal i.e. it has colour from bottom to top. Synthetics are also generally lower in density and are often highly porous.
Opals of all varieties have been synthesized experimentally and commercially. The discovery of the ordered sphere structure of precious opal led to its synthesis by Pierre Gilson in 1974. The resulting material is distinguishable from natural opal by its regularity; under magnification, the patches of colour are seen to be arranged in a “lizard skin” or “chicken wire” pattern.
Furthermore, synthetic opals do not generally fluoresce or incandesce under ultraviolet light however with new technologies we have seen some evidence of incandescence in synthetic opal which is much shorter than in homogenous natural opal, therefore can lead to inconclusive identification of type. If in any doubt, take your sample to a gemmologist for specific testing.
Two notable producers of synthetic opal are Kyocera and Inamori of Japan. Most so-called synthetics, however, are more correctly termed “imitation opal”, as they contain substances not found in natural opal (such as plastic stabilizers). The imitation opals seen in vintage jewellery are often foiled glass, glass-based “Slocum stone”, or later plastic materials.
Various angles of the same Gilson synthetic opal specimen. The static nature of the pattern is seen in the left and middle photos, and the telltale vertical columns of colour in the right hand photo.
Imitation – these are “look alikes” and can simulate a natural opal’s play of colour. Most commonly NZ paua shell/abalone and Canadian ammolite.
Treatments
Andamooka Matrix
Opal is made up of spheres of silica dioxide molecules which are generally half the size of a wavelength of visible light – approx. 150 to 300 nanometres in diameter.
In ‘precious opal’, the spheres are relatively consistent in size and stacked evenly in a three dimensional array. This creates lattices that diffract white light into different colours of the visible spectrum.
Smoked and Dyed Black Opal
Over the past decade, the market has seen an influx of Ethiopian opals, many of which are treated to mimic the famed black opals of Australia. These hydrophane opals are often smoked or dyed to achieve the desired black appearance. Unfortunately, these treatments are frequently undisclosed, making it crucial for buyers to exercise caution.
Because smoked Ethiopian opal is a treatment to a natural homogenous stone, it is hard to detect to the casual observer. Ethiopian opal has a very high water content upon mining – around 20%. Therefore it will quickly absorb skin oils, sweat, perfumes, lotions etc and change colour and body tone. A simple test can be carried out by trying to stick the opal to the end of your tongue – if it adheres, it’s not Australian! But this is not a definitive result.
If in any doubt, take your sample to a gemmologist for specific testing.