First, if you question whether it is real, take it to a jeweler and let them investigate it. Often the diamond will come with a certificate of authenticity. What they look at are as follows:
- Diamond Color: The degree to which a diamond is colorless..the paler the better
- Diamond Clarity: Presence of inclusions in a diamond..how clear it is
- Diamond Carat: Weight of a diamond...size
- Diamond Cut: Angles and proportions of a diamond...shape
All this equates to value of the diamond, but they can tell you whether you have a Cubic Zirconia or a diamond using these values.
You can also help to determine whether a diamond is real by sinking it in water. If the diamond appears invisible in water, this indicates it is genuine.