If you want to know if your diamond is a real natural stone, you can do five tests at home. These can help distinguish between diamonds, cubic zirconia (CZ), and artificial diamonds such as moissanite.
The last three tests work better if you have a loose stone. If the stone fails all five tests, your diamond is likely fake. However, if the stone passes at least one test, there is a chance you have a real diamond. Take the suspect diamond to your local jeweler or gemologist for an official jewelry appraisal. It is difficult to know if a diamond is natural without a gemologist's help. For instance, moissanite is a diamond simulant that is nearly indistinguishable from natural diamonds without gemological testing.
Position the stone close to your mouth and breathe onto the flat surface of the diamond. Imagine you are breathing into a mirror to make it foggy. Diamonds immediately dispense heat, so instead of remaining cloudy, the diamond would become instantly transparent. Try this same test on cubic zirconia. The surface of the CZ will continue to be foggy. This test will help you determine if you have a CZ instead of a diamond, but the test will not tell the difference between moissanite and diamond.
If you have a jewelry loupe at home as well as a basic understanding of diamonds, this test will help you quickly pick out lower grade diamonds. Most diamonds have some inclusions or flaws. If you inspect your stone under magnification in good light, you might notice a small piece of carbon or a white spot that looks like a feather. Cubic zirconia and other synthetic diamonds like moissanite do not have these types of inclusions and are internally flawless.
The presence of inclusions will help you determine if your diamond is real. However, if you don't see any flaws, do not use this test as the deciding factor. Diamond-specific inclusions will indicate a real diamond, but there may be instances where you are inspecting an internally flawless diamond that is without any inclusions.
For the scratch test, take the stone and try to scratch a piece of glass with it. This should be done with caution because it is possible to damage or chip a diamond. Because diamonds are ranked hardest on the Mohs scale, a real diamond should scratch glass. If your stone does not leave a scratch on the glass, it is most likely a fake. If it does leave a scratch, proceed with some additional tests because some synthetic diamonds will also scratch glass.
The newspaper test works well when you have a slightly larger stone. If you place cubic zirconia over a piece of a finely printed newspaper, you should be able to read the print underneath the stone. Diamond, however, refracts the light in such a way that you cannot read the print below it. If you don't have a piece of newspaper lying around, take a piece of paper and draw a small dot on it. You will not be able to see the dot through a real diamond.
Drop a diamond into a glass of water. This will only work if your diamond is loose. Due to the differing densities, a real diamond will sink to the bottom of the glass where most diamond simulants will float.