This diamond is real, you wouldn't know if it wasn't.
There are two environments where you may have to tell the difference between a diamond and a cubic zirconia:
- Mounted in a setting on a ring or other piece of jewelery
- While the gem is loose and not attached to anything
It is much easier to tell the difference between a CZ and a diamond when they are loose.
Once such test involves making a dot on a piece of paper with a pen. Take the gem and put it face down on top of the dot. If the stone is a CZ, you will see a circular reflection. If the stone is a diamond, the reflection will be diffused to the point that you can hardly see it.
Of course, such a test is impossible to perform on a mounted stone.
Other ways to check if you have a cubic zirconia or a diamond are to look at it under magnification and look for scuffs or scratches. Diamonds are extremely hard and can only be damaged by other diamonds. CZs are softer (but still pretty hard) and are more likely to get scuffs and scratches. Again, this requires a loupe (jeweler’s magnification tool) or microscope, and isn’t really perceptible with the naked eye.
And if you have a CZ that gets scratched, it’s easy to replace as they are extremely inexpensive. Even diamonds will occasionally chip or scratch, and they will be much, much more expensive to replace.
A third way to tell the difference between cubic zirconia and diamond is to give it the “fog test.” Breathe on the stone like you would breathe on the lens of your glasses before you clean them. If it stays fogged for a few seconds, it’s a CZ. If the fog vanishes instantly, it’s a diamond.
Notice that all three of these methods require close inspection of the stone. No one that you randomly talk to on the street is going to remove the stone, have a loupe with them, or breathe on it in order to test it. As far as casual visual appearance goes, it is quite difficult to distinguish between CZ and diamond, especially when the stone is mounted on a setting on a piece of jewelery.