Ask for a certificate
If a piece is accompanied by a certificate of authenticity, it is very likely the real deal. You should review the certificate carefully to determine the credibility of the issuing authority. Certificates are issued by various authorities and independent appraisers. Ensure the certificate is issued by a reputable organization such as the American Society of Appraisers.
Breath on the surface of stone with your mouth and see if fogs up. Since diamonds are very good conductors the moisture clears up almost instantaneously. Other stones such as quartz, glass and zirconium stay hazed for quite a few moments. Moissanite passes this test quite convincingly so you may have to go a step further and perform a full thermal conductivity to differentiate it from real diamond.
Examine the facets
Diamonds can be cut more precisely than CZ and other simulates. If you examine the edges between the facets of a real diamond with a magnifying glass, they should be sharp and crispy. The lines on fake diamonds are usually rounded and have abrasions and dents.
The shadow cast by a stone can also help you differentiate between a real and a fake diamond. The shadows of real diamonds are usually grayish with no rainbows. If you shine a light through the stone, you should see a bright halo around the rim again with no rainbow. Rainbows in the reflection or shadow are an indication that the stone is either fake or of poor quality.
See through test
If you turn the stone upside down and place it on a piece of print you shouldn’t see any print through it at all if it is real. The reason for this is that diamonds refract so much light. With fake diamonds you can see the print clearly. If the diamond is not cut proportionately, it may allow some light to pass across it so this test isn’t very reliable as a way to tell natural diamonds.
Look for impurities
During the natural formation process of diamonds various minerals and crystals are embedded in the diamond. These inclusions result in visible spots or flaws on the stone. Rather than being a sign of low quality, these flaws are actually a stamp of authenticity. Fake diamonds and enhanced diamonds usually don’t have these imperfections.
Weigh your gems
Using a precision scale, you can weigh your stones to bust the fake. Cubic zirconia for example weighs more than 50% heavier than authentic diamonds of the same size and shape.