A lot of customers struggle leaving in their jewelry in a store they are not familiar with. They have either had a really bad experience or heard about someone having a bad experience while they left their valuables unattended; well luckily diamonds are very easy to identify, if you have all the right tools and information.
It is nearly impossible for one diamond to match another one. It is often compared to fingerprints as each one is unique in their own way. If your diamond has been papered by a gemological laboratory, all of the information you need has already been obtained to identify your stone. If it hasn't, your jeweler should have the proper tools to give you the information that you need to help identify your diamond.
You can start by looking at your diamond externally to see if there are any marking that you can see with your naked eye. After you have carefully inspected your stone, pick some of the imperfections to use as marks and put down a description of their appearance and location on paper. Not all imperfections in diamonds are visible to the naked eye therefore louping (using x10 magnifying glass) every diamond you have before it is left unattended is a must to help identify. For some higher quality diamonds it is more difficult to pick out the imperfections even using a jeweler's loupe. So you can simply measure your diamond length by width using a mm gauge to help determine precise measurements of your diamond. It is truly that simple.
If your diamond is certified by GIA, it will come with a document similar to the one posted above. The certification will give you a precise measurements of its length, width & depth, carat weight, and also an exact plot of your diamond from the view of its table and pavilion.
Every diamond certified by GIA is assigned a report number. Some of the stones will have a lasered inscription the report number to the girdle that can easily be identified using a loupe. Knowledge is power, the more you know about your diamond, the easier it is to identify.