Read Through Test
If the diamond is un-mounted or loosely mounted, turn it upside down on a printed paper. Now check whether you can read the text through the stone. If not, it will be a real diamond.
Diamonds are good conductors of heat. To test this, try breathing on the stone. If the fog disperses immediately it is a real diamond. On the other hand, a fake diamond will remain foggy for a few seconds longer.
You can perform this test provided that you can access a jeweler’s microscope or loupe. Look at the stone through a microscope and see the number of facet lines. If there is only a single facet line for each, then it is a real diamond. Two lines mean it is a fake one.