Telling the difference between a real opal and a synthetic stone can be tricky because opals come in a rainbow of hues and appear in jewellery in several forms.
Opals are available as faceted gems, cabochons (smooth stones with rounded tops), and partially man-made doublets and triplets. A doublet is a slice of genuine opal with an artificial backing. A triplet has a third layer, a clear dome on top.
Doublets and triplets are fairly easy to spot. Synthetics can be tougher to identify but tests exist to determine whether your opal is a precious gem or a cheap imitation.
Learn what genuine opals look like. Precious opals are translucent gems that display vibrant multicoloured flashes. They include black opals, which have a black, blue or green body colour; and white opals, whose body colour ranges from white to creamy yellow. Mexican fire opals are another gem variety. Transparent to translucent, they come in solid hues of yellow, orange or red, and seldom show the colour play of their precious cousins. Opals can also have bands of different colours.
Check the opal's base tone. If the body of a stone giving off vivid multicoloured flashes is transparent or white, it is most likely a genuine crystal or white opal. If the base colour is brilliant yellow, orange, red or a combination of these hues, without flashes of other colours, it is probably a genuine fire opal.
If the stone's body tone is dark -- black, blue or green -- it could be a genuine solid opal, a doublet or a triplet.
Check the side of the opal stone for artificial layers. Doublets have a black or colourless backing with a thin slice of opal, while triplets have a black backing, a thin slice of opal and a plastic or quartz capping. If you spot any backing on the stone, then it is not a solid opal.
Check the top of the opal stone for a glassy surface. The clear cap on a triplet can form a highly reflective surface.
Check for a snakeskin pattern that displays bright colours in large patches. The pattern, which is often a little too perfect, is an indicator that the stone is a synthetic opal.
Check the top of the stone. If the stone has a cloudy look to it, you are most likely looking at a triplet or doublet. After the stone has been in water for a long period of time, the glue that holds the backing and opal together starts to deteriorate and becomes hazy.
If you're still unsure about the authenticity of your opal, ask a gemologist to test it for you. Many jewellers have gemologists on their staffs. These professionals can test for specific gravity, refractive index and other factors that are hard to evaluate without special equipment.