I've heard that certain synthetic stones appear different under ultra violet light for example, but I don't know if this is something the manufacturers do to distinguish their product from the naturals or if they just can't perfectly mimic the natural crystallography.
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Yes there are many tests used to differentiate natural from synthetic gemstones. The main tool is the personal experience of the tester closely followed by the microscope, the minute inclusions within the stones are very characteristic in most cases. Beyond this there is a whole battery of tests ranging from simple optical instruments like the polariscope to x-ray diffraction analysis. Some synthetics are amazingly good but to date there have been none that can defeat a determined, expert tester. The producers of synthetic stones do not add anything to their creations to make them easy to detect, in fact their aim is exactly the opposite, to make a product that will pass as the real thing even under close inspection
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they look fake and will fail tests the prove real ones