Fake opals can be fairly difficult to identify because the varieties and types of real opals are so diverse. However, there are some good ways to get a good idea about the real value of an opal by doing a little research and making some skilled inquiries of the jeweler selling the gem. While you should always consult a licensed, independent jeweler before making a major jewelry purchase of any type, if you do not have this option and want to buy the jewelry anyway, then factor these considerations into your negotiations before making the purchase.
Check the country of origin. Most real opals are mined in Australia, which is the opal capital of the world. Russia and Hong Kong make very high quality fake opals that some unscrupulous jewelers try to pass off as the real thing, but if your opal does not come from Australia then you should seriously suspect its validity.
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Inspect the symmetry. If the opal is a natural stone, then even with cutting and polishing it may not be entirely symmetrical. If the opal is a perfect circle or oval, then you should have a licensed jeweler inspect it before you make the purchase.
Inspect the opal in strong, white light. Do not use a fluorescent light, as this can make the opal appear to have more facets than it actually does and can give the impression of additional colors. If the opal appears to have multiple layers of color, then this is a good indication that it is a real stone. However, if an opal only appears to have colors immediately below the surface, it may be a fake.
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Check the price. A real opal, even a small one, will usually cost over $100. If an opal in a jewelry setting costs only $20 or $40, then you should suspect the bargain.
Work with an independent jeweler. Anytime you are considering making a major jewelry investment, you should work with a jeweler or certified appraiser that is not affiliated with the sales team that you are working with also. This will help you make an objective decision. While you may have to take precautions to make sure that everyone is comfortable with the presence of the additional inspector, if the vendor has nothing to hide then they will gladly allow an outside party to inspect the stone.