How do you know if a diamond is real?

There are a number of different ways to tell. The quickest and easier is the fog test: a real diamond will disperse the heat from your breath immediately, the setting: such as 10k, 14k or 18k stamps inside the setting, anything with a C.Z (cubic zirconia) usually indicates that the diamond is not real, a jewelers loupe and also if it has rainbow reflections if you look at it from the top. You can also take it to a jeweler and get it evaluated.

Dropping a stone to see if it will smash could be a very expensive way of testing it. Some cubic zirconiums--the stones most often substituted for diamonds--can be a little pricey.

There are two relatively easy ways to test a diamond.

First, run it across glass. I don't advise using the window of your dad's Beemer for this. Use a cheap drinking glass, or buy a small piece of window glass from a hardware.

Second, all diamonds that are purchased from a retailer or wholesaler today come with a certificate of authenticity. It will have a number, and the quality listed on this certificate.

Failing either of these two options, take it to a reputable jeweler and have the stone checked.

Your jeweler can test the stone to determine whether or not it is a diamond. The jeweler will use a probe.

At home, you can inspect the stone under a 10x loupe: if you see any chips or fixed waves, the stone is not a diamond.
Ask your local jeweler, and they will usually check for free.

Of course, an easy layman way to find out is to look at the size of the diamond in relation to the social status of the person wearing it.

If a woman at K-mart is wearing what appears to be a one carat diamond ring, it's probably fake.