One of the easiest tests is to lay the diamond table down on a piece of printed paper (say a newspaper). Pretty much anything which is not a diamond will allow you to read the letters clearly; a diamond has such a high RI that the letters will appear blurred. However, this won't work with some diamond simulants.

For a definitive answer, go to a jeweller; they should be able to test it and tell you at least whether it is a diamond or not.

BTW - a 2 mm diamond is probably about 3 points (0.03 ct); it's not worth a lot, so it may well be real, even if the jewel is not very important/expensive

thank jou guys, I did the newspaper test and my diamond didnt let the letters trough so I think that its a real one 

Well, since it's a little heirloom diamond, and it worked itself loose, would you think about setting in in a ring perhaps? It would be nice if you could wear it, maybe.

 I know you said it's small, but a gent's ring with a small diamond accent can look fantastic. Or maybe set it into a band with some coloured diamonds of a similar size? Just a thought, but it might be a nice way to start your coloured diamond collection without spending a fortune.

Glad you managed to catch the diamond when it fell out!

Jen

Lucky you managed to catch the diamond when it fell out! If you're still not sure, here are a few more tests you can try: breath on the stone and see if it fogs up. It it's a real diamond it shouldn't have any condensation on it. Also, hold it up to your tongue for a moment. If it stays cold - and makes your tongue cold - it's probably a real diamond because diamonds are great heat conductors and absorb temperature.