Real diamonds are more transparent and efficient at conducting heat than moissanite and cubic zirconia stones. Under magnification, real diamonds refract differently than their fake counterparts and have minute physical differences, such as natural inclusions and a different texture.
One easy way of determining a real diamond from a fake is turning it upside down and attempting to read a line of text through it. A real diamond has a slightly frosted, grainy look that obscures the text. The clearer and more visible the text, the less likely the diamond is real.
Another quick way of testing a diamond is by breathing on it. The fog caused by warm breath dissipates instantly on a real diamond due to its efficient heat conductivity, but the heat lingers for a few seconds longer on a fake stone. If a diamond tester or thermal conductivity probe is available, it can detect how quickly a stone dissipates heat and separate a fake gemstone accurately. However, the diamond tester does not work on synthetic moissanite.
To confirm that a gemstone is a diamond and not a synthetic moissanite, examine it under a jeweler's microscope, and pay attention to the bezel or star facets of the crown. A diamond is singly refractive and displays only one line for each facet, while a moissanite is doubly refractive.