5 ways to spot a fake diamond

This is an easy test, since diamonds are one of the world's hardest materials and won't be scratched by the rough surface. "If it's a diamond, it will remain perfect. If it's a cubic zirconium, it will scratch it up," Hirsch said.

Breathe hot air on your diamond the same way you would if you were fogging up a bathroom mirror.

"A fake diamond will fog up for a short period of time, whereas a real diamond will not because it won't retain the heat," Hirsch explained.

The way that diamonds reflect light is unique: Inside the stone, the diamond will sparkle gray and white (known as "brilliance") while outside of the gem, it will reflect rainbow colors onto other surfaces (this dispersed light is known as "fire").

A fake diamond will have rainbow colors that you can see inside the diamond.

"People have a misconception that diamonds sparkle like a rainbow, but they don't," Hirsch said. "They do sparkle, but it's more of a gray color. If you see something with rainbow colors [inside the stone], it could be a sign that it's not a diamond."

Diamonds are so sparkly because of the way they refract and bend light. Glass, quartz, and cubic zirconium may mimic a diamond's brilliance, but they have much lower refractive indexes.

This means that if your stone isn't in a setting, you can place it over a newspaper, and the light will scatter inside the real diamond and prevent a black reflection. A fake diamond will let the black shine through, and you may even be able to read a word through it, depending on the size of the fake stone.

If your diamond is mounted, make sure you can't see through it to the mount itself — that's a very bad sign.