Check the ‘Read-Through’ effect:
If the stone is loose or unmounted you can turn it upside down on a printed piece of paper. If you cannot read the text through the stone it may be a real diamond. If the letters are visible, it is probably not a genuine diamond. This test works best with properly cut round brilliant stone.
Try the ‘Fog Test’ (or ‘Breath Test’):
Since diamonds are efficient heat conductors, once you breathe on a real diamond the fog should disperse immediately. If the stone is a fake diamond the top will stay foggy for a few seconds longer. This test works best with a clean stone.
Test with a Thermal Conductivity Probe (also known as a diamond tester):
As mentioned, diamonds are efficient heat conductors. Using the thermal test separates most fake diamonds from real diamonds by detecting the rate the stone disperses heat. This test works on most gemstones, with an exception being synthetic moissanite. A synthetic moissanite stone may register as a real diamond with the thermal test.
If you have access to a jeweler’s loupe or microscope there are many observations you can make to help determine if the stone is a real diamond. If the stone passes the heat test, you will want to confirm that it is a real diamond and not a moissanite. Using magnification, look at the gemstone through one of the bezel or star facets on the crown. If the stone shows doubling, which means it looks like there are two of each facet line, then it is probably a moissanite. If you see only one of each facet line (singly refractive) and it passed the heat test then it may be a natural diamond.