Blowing breath onto a diamond, rubbing it with sandpaper, observing how it sparkles and how it refracts light are ways to determine its authenticity. Studying a diamond's imperfections is another technique, though this requires a special jeweler's magnifying device called a loupe.
Breathing hot air onto a diamond helps determine if it is real. If the diamond fogs up, it is fake. Real diamonds do not retain heat, whereas fake diamonds do. Diamonds are one of the hardest materials in the known world, and sandpaper cannot scratch them. If rubbing a diamond with sandpaper scratches it, the diamond is not real.
Diamonds reflect light in a very specific way. When held up to a light, a diamond's internal brilliance appears gray and white, while light dispersed by the diamond appears as rainbows on nearby surfaces. A diamond that has rainbows glistening inside the stone when it is held up to the light is a mere pretender.
Diamonds tend to refract and bend light in a unique manner as well. The measurement of this quality is called the refractive index. The refractive index of a real diamond is much higher than that of a fake diamond. When superimposed over newspaper print, a real diamond bends the light so intensely that the black cannot shine through.
Observing a diamond through a loupe reveals certain characteristics typical of real diamonds. Sharp edges and carbon imperfections, which may appear as tiny blemishes, black or white spots, and tiny fractures, are such telltale characteristics.