Things You Will NeedThe following tips, or an independent appraiser.
Step 1Simple Tests.
The Fog Test - Diamonds dissipate heat rather quickly, so the fogging that appears after blowing on the stone will quickly disappear.
The Transparency Test - Diamonds have a high refractive index, which means they sharply bend light so that it doesn't freely pass through to the other side; this is what gives diamonds their brilliant luster. This quality also makes it impossible, unless the diamond has a disproportionate cut, to read any print through the diamond. To check if a loose diamond is genuine, place it on a piece of newspaper to see if the print can be read through the stone, if you can read the print it's a fake.
Rainbow colored reflections - Diamond's refractive index also cause them to sparkle in shades of grey, so stones that have rainbow sparkles are most likely fakes.
Exterior Damage - As a result of being formed under extreme high heat and pressure deep within the earth, diamonds are the hardest bulk material known to man, that is if hardness refers to scratch resistance and not to structural rigidity. As a matter of fact a diamond can be easily broken apart by a single blow of a hammer. Its scratch resistance is one reason diamonds have become popular precious stones for use in jewelry that's worn on a daily basis such as wedding rings. Because of their scratch resistance, diamonds are less likely to have scuffs and nicks
Check the setting and mount - It is unlikely that a genuine diamond would be set in inferior material. The setting is likely to be stamped with designations (10k, 14k, 18k, 585, 750, 900, 950 PT, Plat) that may indicate that real gold or platinum was used. A 'C.Z' designation is a give away that the diamond is fake.
Step 2Ask an Expert.
Weigh the stone - Cubic zirconia weigh approximate 55% more that a real diamond. You can use a carat weight scale to get an estimate of what a real diamond should weigh, in proportion to its size.
Have the diamond appraised - Work with a reputable jeweler to have the diamond appraised and certified. As far as is possible, ensure that the appraiser is from an impartial grading authority such as the Gemological Institute of America or the Jewelers Association of Australia. If the appraiser is independent, check that he is a member of the American Society of Appraisers.
Use a diamond tester. These are readily available and can easily detect fake diamonds. Unless you are planning on becoming a gemologist you might not want to buy a loupe or a microscope, so have an appraiser do this for you.