This is how to identify a real diamond using a few safe and simple tests. When you put a loose diamond on its head on the word of a text, the letters are unreadable. A diamond lights up blue under a UV light source. When you breathe on a clean diamond, it does not fog up. A real diamond sinks immediately in a glass of water, while a fake diamond floats or sinks slowly. When a diamond is heated for 30 seconds (e.g., with a cigarette lighter) then put in a glass of cold water, it will not shatter.