Sometimes we get asked by a customer if the stone that they found tucked at the bottom of their jewellery box is a real diamond or not.
One very quick way to rule out many fakes is to try the newspaper test. Diamonds are highly dispersive, and have a high refractive index. This means that they bend and break up light very well, more than many other stones that are sometimes mistaken for a real diamond.
To do the test, simply place the diamond face down on a piece of newspaper text. See if you can read the text through the diamond. In the first photo, a piece of glass is shown. The text is clearly legible, as glass only barely bends light.
Finally, a diamond. The text cannot be read through the stone.
Please note that this text is a good rule of thumb, but only works with modern round brilliant cut diamonds, and is not definitive; unusual or non-standard cut proportions can be misleading! If in doubt, have the stone appraised by a professional gemmologist.