Last night, my couch spit out a diamond. Seriously! I Googled "Is this diamond real or fake?", as dollar signs flashed though my head.
One test is to make a dot on a white piece of paper. Place the stone upside down over the dot, and if you can see the dot through the stone as a series of broken lines, you may have a real diamond. If the dot instead appears exactly as the dot you drew, you've got a fake.