Created from the fusion of silicon dioxide and pressurised water, fiery opals have been valued by many cultures, including the ancient Greeks and Romans. Ninety-five per cent of the opals mined today come from Australia, where the opal enjoys the distinction of being the National Gemstone of Australia. The remaining five per cent of opals come from the United States (Nevada and Idaho), Brazil and Mexico. Especially rare and valuable are Peruvian Blue Opals. With great value comes a great number of forgeries, making it important to check your opal for the telltale signs of a fake.
Hold your opal up to a bright light, or take a photo of your opal using the brightest flash setting on your camera. Natural opals will turn translucent, while fake opals will remain cloudy and opaque. This will be visible to the naked eye, or will be visible in the photo of your stone. If you see specks of dust or dirt, you are looking at a fake opal with dirt trapped between the layers. For opals that are mounted in jewellery, the photo technique is especially useful.
Turn your stone over and examine the backing. If your stone has a dark navy or black backing that is evenly joined to the stone, you are most likely looking at a fake doublet or triplet opal. Doublet opals are made by joining a piece of low grade or synthetic opal to a piece of ironstone or other organic material. Triplet opals are made following the same process, but usually using a cheaper material like plastic and contain an additional layer on top of the stone. While natural opals are found attached to other stones, the join is usually uneven and rough. The line between a fake opal and its backing is usually regular and smooth
Examine the colour of your opal. Oftentimes fake opals are dyed to enhance their colour, leading to gaudy, overly bright shades of blue opal. Fake opals will also be uneven in colour, with patches of concentrated colour and uneven tone. This is detected by exposing the stone to bright light.
Look at the sides of your opal stone. If you can see through the top of the stone, you are looking at a triplet opal that has been topped with a disc of glass or plastic.
To ensure that you purchase a genuine opal, buy stones from jewellers and vendors who have been certified by a reputable firm like the American Gem Society or the Gemological Institute of America. These jewellers have undergone rigorous testing and are held to exacting standards, and stones purchased from these vendors can be trusted as genuine.